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Practice › LeetCode › LeetCode (archived) › LeetCode 08xx ›

LeetCode 897: Increasing Order Search Tree

Problem Restatement We are given the root of a binary search tree. We need to rearrange the tree in inorder order so that: Requirement Meaning New root The leftmost node of the original tree Left child Every node must have no left child Right child Each node may have one right child Order Nodes appear in increasing order The result should be a right-skewed tree, like a sorted linked list...
leetcode tree binary-tree binary-search-tree dfs inorder-traversal May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 08xx ›

LeetCode 872: Leaf-Similar Trees

Problem Restatement We are given the roots of two binary trees: root1 root2 For each tree, collect all leaf values from left to right. A leaf node is a node with no left child and no right child. Two trees are leaf-similar if their leaf value sequences are the same. Return True if the two trees are leaf-similar. Otherwise, return False . Input and Output Item Meaning Input root1 ,...
leetcode tree binary-tree dfs May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 09xx ›

LeetCode 964: Least Operators to Express Number

Problem Restatement We are given two integers: x target We may build expressions using only the number x and the operators: + - * / Division is real division, and standard operator precedence applies. We cannot use unary negation. So: -x is not allowed directly. We want to express target using the minimum number of operators. Return that minimum number. The official constraints are: Constraint Value 2 <= x <=...
leetcode math dynamic-programming dfs memoization May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 797: All Paths From Source to Target

Problem Restatement We are given a directed acyclic graph, also called a DAG. The graph has n nodes labeled: 0, 1, 2, ..., n - 1 The graph is represented as an adjacency list. graph[i] contains every node we can visit directly from node i . We need to return all possible paths from node 0 to node n - 1 , in any order. The graph is guaranteed to...
leetcode graph dfs backtracking dag May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 250: Count Univalue Subtrees

Problem Restatement We are given the root of a binary tree. Return the number of uni-value subtrees. A uni-value subtree means every node in that subtree has the same value. An empty tree has 0 uni-value subtrees. The constraints say the tree has between 0 and 1000 nodes, and each node value is between -1000 and 1000 . Input and Output Item Meaning Input Root of a binary tree Output...
leetcode binary-tree dfs recursion May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 248: Strobogrammatic Number III

Problem Restatement We are given two strings, low and high , representing two integers. Return the number of strobogrammatic numbers in the inclusive range: low <= num <= high A strobogrammatic number looks the same after being rotated 180 degrees. The numbers are represented as strings because the range can be large. The constraints say 1 <= low.length, high.length <= 15 , low and high contain only digits, low <=...
leetcode recursion string dfs May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 00xx ›

LeetCode 46: Permutations

Problem Restatement We are given an array nums containing distinct integers. We need to return all possible permutations of the array. A permutation is one possible ordering of all elements. The answer may be returned in any order. The official problem states: given an array nums of distinct integers, return all possible permutations. Example: nums = [1, 2, 3] The possible permutations are: [ [1, 2, 3], [1, 3, 2],...
leetcode array backtracking dfs May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 08xx ›

LeetCode 865: Smallest Subtree with all the Deepest Nodes

Problem Restatement We are given the root of a binary tree. The depth of a node is its shortest distance from the root. A deepest node is a node with the largest depth in the whole tree. We need to return the smallest subtree that contains all deepest nodes. Since a subtree is represented by its root node, we return the root of that smallest subtree. Another way to say...
leetcode tree binary-tree dfs recursion May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 06xx ›

LeetCode 623: Add One Row to Tree

Problem Restatement We are given the root of a binary tree and two integers val and depth . We need to add a new row of nodes with value val at the given depth . The root is at depth 1 . If depth == 1 , we create a new root with value val . The original tree becomes the left child of the new root. Otherwise, for every...
leetcode binary-tree dfs bfs tree May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 08xx ›

LeetCode 814: Binary Tree Pruning

Problem Restatement We are given the root of a binary tree. Every node has value either 0 or 1 . We need to remove every subtree that does not contain a 1 . A subtree means a node together with all of its descendants. If a subtree has only 0 values, that entire subtree should be removed. Return the root of the pruned tree. The official problem asks us to...
leetcode binary-tree dfs recursion May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 700: Search in a Binary Search Tree

Problem Restatement We are given the root of a binary search tree and an integer val . We need to find the node whose value equals val . If the node exists, return that node. Returning the node means returning the entire subtree rooted at that node. If the node does not exist, return null . The official statement asks us to return the subtree rooted at the node whose...
leetcode tree binary-search-tree dfs May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 08xx ›

LeetCode 863: All Nodes Distance K in Binary Tree

Problem Restatement We are given the root of a binary tree, a target node, and an integer k . We need to return the values of all nodes whose distance from the target node is exactly k . Distance means the number of edges on the path between two nodes. The answer can be returned in any order. Input and Output Item Meaning Input root , the root of the...
leetcode binary-tree graph dfs bfs May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 711: Number of Distinct Islands II

Problem Restatement We are given a binary grid. Value Meaning 1 Land 0 Water An island is a group of connected land cells using 4-directional movement: up down left right We need to count how many distinct island shapes exist. Two islands are considered the same if one can be transformed into the other using: Allowed Transformation Rotation Reflection Translation This means two islands are identical if we can rotate...
leetcode grid dfs geometry hashing May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 08xx ›

LeetCode 886: Possible Bipartition

Problem Restatement We are given n people labeled from 1 to n . We are also given a list dislikes , where each pair: [a, b] means person a dislikes person b , and person b dislikes person a . We need to split everyone into two groups so that no pair of people who dislike each other are in the same group. Return true if this is possible. Otherwise,...
leetcode graph dfs bfs bipartite-graph May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 06xx ›

LeetCode 695: Max Area of Island

Problem Restatement We are given an m x n binary matrix grid . Value Meaning 1 Land 0 Water An island is a group of land cells connected horizontally or vertically. The area of an island is the number of cells in that connected component. We need to return the maximum island area in the grid. If there is no island, return 0 . The official statement defines islands using...
leetcode array matrix dfs bfs May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 06xx ›

LeetCode 694: Number of Distinct Islands

Problem Restatement We are given an m x n binary matrix grid . A cell with value 1 is land. A cell with value 0 is water. An island is a group of land cells connected horizontally or vertically. We need to count how many distinct island shapes exist in the grid. Two islands are considered the same if one can be translated to match the other. Translation means moving...
leetcode array matrix dfs hash-set May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 785: Is Graph Bipartite?

Problem Restatement We are given an undirected graph as an adjacency list. graph[i] contains all nodes connected to node i . We need to return True if the graph is bipartite. A graph is bipartite if its nodes can be split into two groups such that every edge connects nodes from different groups. Equivalently, we need to check whether we can color every node using two colors so that no...
leetcode graph dfs bfs coloring May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 06xx ›

LeetCode 690: Employee Importance

Problem Restatement We are given a list of employees. Each employee has: Field Meaning id Unique employee ID importance Integer importance value subordinates List of direct subordinate IDs Given an employee ID, return the total importance of that employee and all their direct and indirect subordinates. The official statement asks for the sum of the importance values for the given employee, plus every employee under them in the hierarchy. Direct...
leetcode hash-map dfs bfs tree May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 784: Letter Case Permutation

Problem Restatement We are given a string s . Each alphabet letter may be changed independently to lowercase or uppercase. Digits must stay the same. Return all possible strings we can create. The output may be returned in any order. Input and Output Item Meaning Input A string s Output A list of all possible letter-case permutations Letters Can become lowercase or uppercase Digits Must stay unchanged Constraint 1 <=...
leetcode string backtracking dfs recursion May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 783: Minimum Distance Between BST Nodes

Problem Restatement We are given the root of a binary search tree. We need to return the minimum difference between the values of any two different nodes. Because the tree is a BST, an inorder traversal visits the node values in sorted order. So instead of comparing every pair of nodes, we only need to compare neighboring values in inorder order. Input and Output Item Meaning Input Root of a...
leetcode tree binary-search-tree dfs inorder-traversal May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 06xx ›

LeetCode 687: Longest Univalue Path

Problem Restatement We are given the root of a binary tree. We need to return the length of the longest path where every node on the path has the same value. The path may or may not pass through the root. The length of a path is counted by number of edges, not number of nodes. The official constraints allow an empty tree, up to 10^4 nodes, node values from...
leetcode tree binary-tree dfs recursion May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 06xx ›

LeetCode 638: Shopping Offers

Problem Restatement We are given item prices, special bundle offers, and the exact number of each item we need to buy. Each item i has an individual price: price[i] Each special offer contains quantities of items plus a final bundle price: special[j] = [count_item_0, count_item_1, ..., offer_price] We can use each special offer any number of times. The goal is to pay the minimum total cost while buying exactly the...
leetcode dynamic-programming dfs memoization backtracking May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 06xx ›

LeetCode 617: Merge Two Binary Trees

Problem Restatement We are given two binary trees: root1 and root2 . We need to merge them into a single binary tree. The merge rule is: Case Result Both nodes exist Sum their values One node is null Use the non-null node Both null Result is null We return the merged tree as the new root. The official problem defines merging two binary trees by summing overlapping nodes and using...
leetcode tree binary-tree recursion dfs May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 756: Pyramid Transition Matrix

Problem Restatement We are given a bottom row of blocks. Each block has a color represented by one letter. We are also given a list of allowed triples. A triple has length 3 . For example: "ABC" means: A B C If two adjacent blocks are A and B , then we may place C on top of them. Starting from the given bottom row, we need to decide whether...
leetcode backtracking dfs memoization hash-table string May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 514: Freedom Trail

Problem Restatement We are given a circular ring represented by a string ring . We are also given a string key . The ring starts with ring[0] aligned at the 12:00 position. To spell each character in key , we may: Operation Cost Rotate the ring one position clockwise 1 Rotate the ring one position counterclockwise 1 Press the center button to select the current character 1 For each character...
leetcode dynamic-programming dfs memoization string May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 590: N-ary Tree Postorder Traversal

Problem Restatement We are given the root of an N-ary tree. An N-ary tree is a tree where each node may have any number of children. We need to return the postorder traversal of the tree's node values. In postorder traversal, we visit: All children from left to right. The current node after its children. The input may be empty. If the tree has no nodes, return an empty list....
leetcode tree dfs stack recursion May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 508: Most Frequent Subtree Sum

Problem Restatement We are given the root of a binary tree. For every node, define its subtree sum as the sum of all values in the subtree rooted at that node. This includes the node itself and all of its descendants. We need to return the subtree sum value or values that appear most frequently. If several subtree sums have the same highest frequency, return all of them in any...
leetcode binary-tree tree dfs hash-map postorder-traversal May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 589: N-ary Tree Preorder Traversal

Problem Restatement We are given the root of an N-ary tree. An N-ary tree is a tree where each node can have any number of children. We need to return the preorder traversal of the tree's node values. In preorder traversal, we visit: The current node. Its children from left to right. The number of nodes is in the range [0, 10^4] , node values are between 0 and 10^4...
leetcode tree dfs stack recursion May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 753: Cracking the Safe

Problem Restatement We have a safe protected by an unknown password. The password has length n . Each digit can be one of: 0, 1, ..., k - 1 When we type digits into the safe, it checks the most recent n digits after every key press. For example, if the password is: 345 and we type: 012345 then the safe checks: 012 123 234 345 When it sees 345...
leetcode dfs graph eulerian-path de-bruijn-sequence May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 08xx ›

LeetCode 802: Find Eventual Safe States

Problem Restatement We are given a directed graph with n nodes labeled from 0 to n - 1 . The graph is represented as an adjacency list: graph[i] contains every node that node i points to. A terminal node has no outgoing edges. A safe node is a node where every possible path starting from that node eventually reaches a terminal node. If a node can reach a cycle, then...
leetcode graph dfs cycle-detection May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 09xx ›

LeetCode 951: Flip Equivalent Binary Trees

Problem Restatement We are given the roots of two binary trees, root1 and root2 . A flip operation chooses one node and swaps its left and right child subtrees. Two trees are flip equivalent if we can make them equal after doing zero or more flip operations. Return true if the two trees are flip equivalent. Otherwise, return false . The official constraints say each tree has between 0 and...
leetcode tree binary-tree dfs recursion May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 08xx ›

LeetCode 851: Loud and Rich

Problem Restatement We are given n people labeled from 0 to n - 1 . Each person has: Data Meaning quiet[i] Quietness level of person i richer[j] = [a, b] Person a is richer than person b For every person x , we need to find the quietest person among all people who are at least as rich as x . This includes: Person x Everyone directly richer than x...
leetcode graph dfs memoization topological-sort May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 501: Find Mode in Binary Search Tree

Problem Restatement We are given the root of a binary search tree that may contain duplicate values. We need to return all values that appear most frequently in the tree. A value with the highest frequency is called a mode. If more than one value has the same highest frequency, return all of them in any order. The official statement defines the input as a BST with duplicates and asks...
leetcode binary-tree binary-search-tree dfs inorder-traversal May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 582: Kill Process

Problem Restatement We are given a list of process IDs and their parent process IDs. Each process forms part of a process tree. If a process is killed, then all of its child processes are also killed recursively. Given a process ID called kill , return all process IDs that will be terminated. The result may be returned in any order. The official problem defines: pid[i] = process ID ppid[i]...
leetcode tree graph dfs bfs hash-map May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

LeetCode 473: Matchsticks to Square

Problem Restatement We are given an integer array matchsticks . Each value represents the length of one matchstick. We must use every matchstick exactly once. We need to determine whether all matchsticks can form a square. A square has four sides of equal length, so the task is to divide the matchsticks into four groups with the same sum. The official problem asks whether the given matchsticks can make one...
leetcode array backtracking dfs bitmask May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

LeetCode 450: Delete Node in a BST

Problem Restatement We are given the root of a binary search tree and an integer key . We need to delete the node whose value equals key . After deletion, the tree must still be a valid binary search tree. Return the possibly updated root of the tree. The root can change. For example, if we delete the original root, some other node may become the new root. The official...
leetcode tree binary-search-tree dfs recursion May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

LeetCode 399: Evaluate Division

Problem Restatement We are given equations representing division relationships between variables. For example: a / b = 2.0 b / c = 3.0 We are also given queries like: a / c c / a a / e We need to compute the value of each query. If the query cannot be determined from the equations, return: -1.0 The equations are guaranteed to be valid, and there is no contradiction....
leetcode graph dfs union-find weighted-graph May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

LeetCode 449: Serialize and Deserialize BST

Problem Restatement We need to design two methods: Method Meaning serialize(root) Convert a BST into a string deserialize(data) Convert that string back into the original BST The tree is guaranteed to be a binary search tree. There is no required format. Any format is valid as long as deserializing the serialized string reconstructs the same BST. The encoded string should be as compact as possible. The number of nodes is...
leetcode tree binary-search-tree dfs design serialization May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

LeetCode 323: Number of Connected Components in an Undirected Graph

Problem Restatement We are given: n edges There are n nodes labeled: 0 to n - 1 edges describes an undirected graph. Return the number of connected components. A connected component is a maximal group of nodes where every node can reach every other node in the group. The official problem defines the graph as undirected with nodes labeled from 0 to n - 1 . Input and Output Item...
leetcode graph union-find dfs bfs May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

LeetCode 417: Pacific Atlantic Water Flow

Problem Restatement We are given an m x n matrix heights . Each cell represents land height. Water can flow from one cell to another adjacent cell if the next cell has height less than or equal to the current cell. Adjacent means four directions only: up, down, left, right The Pacific Ocean touches the top and left edges. The Atlantic Ocean touches the bottom and right edges. Return all...
leetcode matrix dfs bfs graph May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 298: Binary Tree Longest Consecutive Sequence

Problem Restatement We are given the root of a binary tree. We need to return the length of the longest consecutive sequence path. A consecutive path means each next node has value exactly one greater than its parent. For example: 3 -> 4 -> 5 is valid. But: 3 -> 2 -> 1 is not valid, because the values decrease. The path can start at any node, but it must...
leetcode tree binary-tree dfs May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 297: Serialize and Deserialize Binary Tree

Problem Restatement We need to design a codec for a binary tree. The codec has two operations: serialize(root) deserialize(data) serialize(root) converts a binary tree into a string. deserialize(data) converts that string back into the original binary tree. There is no required string format. The only requirement is that a tree serialized by our method can be deserialized back into the same tree structure. The source statement also clarifies that we...
leetcode tree binary-tree dfs design string May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

LeetCode 465: Optimal Account Balancing

Problem Restatement We are given a list of transactions. Each transaction has the form: [from_person, to_person, amount] This means from_person gave amount dollars to to_person . After all original transactions, some people may owe money and some people may be owed money. We need to return the minimum number of transactions required to settle all debts. The official problem defines each transaction as [from_i, to_i, amount_i] and asks for the...
leetcode array hash-map backtracking dfs May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

LeetCode 366: Find Leaves of Binary Tree

Problem Restatement We are given the root of a binary tree. We need to collect the tree's nodes as if we repeatedly do this: Collect all current leaf nodes. Remove those leaf nodes. Repeat until the tree is empty. Return a list of lists. Each inner list contains the node values removed in the same round. The order inside each round does not matter. The official example is: root =...
leetcode tree dfs postorder binary-tree May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

LeetCode 490: The Maze

Problem Restatement We are given a maze represented by a 2D grid. Each cell has one of two values: Value Meaning 0 Empty space 1 Wall A ball starts at start . The ball can roll in four directions: Direction Up Down Left Right But the ball does not move one cell and stop. Once it starts rolling in a direction, it keeps rolling until it hits a wall. It...
leetcode graph bfs dfs matrix maze May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 270: Closest Binary Search Tree Value

Problem Restatement We are given the root of a binary search tree and a target value. The target can be a floating-point number. We need to return the value in the BST that is closest to the target. If multiple values are equally close, return the smaller value. The tree contains between 1 and 10^4 nodes, node values are in [0, 10^9] , and the target is in [-10^9, 10^9]...
leetcode tree binary-search-tree dfs binary-search May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 216: Combination Sum III

Problem Restatement We are given two integers: k n We need to find all valid combinations of exactly k numbers that sum to n . Rules: Rule Meaning Number range Only numbers 1 through 9 may be used Reuse Each number can be used at most once Combination size Each combination must contain exactly k numbers Combination sum The numbers must add up to n Duplicates The answer must not...
leetcode array backtracking dfs May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 124: Binary Tree Maximum Path Sum

Problem Restatement We are given the root of a binary tree. We need to find the maximum path sum of any non-empty path in the tree. A path is a sequence of connected nodes where each pair of adjacent nodes has an edge between them. A node may appear in the path at most once. The path does not need to pass through the root. The official problem asks for...
leetcode tree binary-tree dfs dynamic-programming May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 145: Binary Tree Postorder Traversal

Problem Restatement We are given the root of a binary tree. Return the postorder traversal of its node values. Postorder traversal visits nodes in this order: left subtree -> right subtree -> root If the tree is empty, return an empty list. LeetCode also includes a follow-up asking for an iterative solution. Examples Example 1: 1 \ 2 / 3 Traversal order: visit left of 1: none visit right subtree...
leetcode Easy tree binary-tree dfs stack May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 144: Binary Tree Preorder Traversal

Problem Restatement We are given the root of a binary tree. Return the preorder traversal of its node values. Preorder traversal visits nodes in this order: root -> left subtree -> right subtree The tree may be empty. In that case, return an empty list. LeetCode also includes a follow-up asking for an iterative solution. Examples Example 1: 1 \ 2 / 3 Preorder traversal: visit 1 go right to...
leetcode Easy tree binary-tree dfs stack May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 114: Flatten Binary Tree to Linked List

Problem Restatement We are given the root of a binary tree. We need to flatten the tree into a linked list in-place. The linked list must follow the same order as preorder traversal: root -> left -> right After flattening: Every node's left child must become None . Every node's right child points to the next node in preorder order. The official problem specifically requires the flattened structure to use...
leetcode tree binary-tree linked-list dfs recursion May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 113: Path Sum II

Problem Restatement We are given the root of a binary tree and an integer targetSum . We need to return all root-to-leaf paths where the sum of node values equals targetSum . Each returned path must: Start at the root. End at a leaf. Have node values summing to targetSum . The official problem asks for all root-to-leaf paths whose sum equals the target. For this tree: 5 / \...
leetcode tree binary-tree dfs backtracking recursion May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 112: Path Sum

Problem Restatement We are given the root of a binary tree and an integer targetSum . We need to return True if the tree has at least one root-to-leaf path whose node values add up to targetSum . A leaf is a node with no children. If no such path exists, return False . The official problem uses this same root-to-leaf path requirement. For this tree: 5 / \ 4...
leetcode tree binary-tree dfs recursion May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 110: Balanced Binary Tree

Problem Restatement We are given the root of a binary tree. We need to determine whether the tree is height-balanced. A binary tree is height-balanced when the left and right subtrees of every node differ in height by no more than 1 . The official problem asks us to return true if the tree is balanced, otherwise false . For this tree: 3 / \ 9 20 / \ 15...
leetcode tree binary-tree dfs recursion May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 212: Word Search II

Problem Restatement We are given: board words board is an m x n grid of characters. words is a list of strings. We need to return all words from words that can be formed on the board. A word can be formed by moving from one cell to another adjacent cell. Allowed moves: up down left right Diagonal moves are not allowed. The same board cell cannot be used more...
leetcode trie dfs backtracking matrix May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

LeetCode 489: Robot Room Cleaner

Problem Restatement We control a robot cleaner inside a room modeled as a grid. Each cell is either: Value Meaning 1 Open cell 0 Blocked cell But the algorithm does not receive the grid directly. Instead, we only control the robot through four APIs: robot.move() robot.turnLeft() robot.turnRight() robot.clean() The robot starts at an unknown open cell and initially faces up. The room boundary is surrounded by walls. All open cells...
leetcode dfs backtracking simulation robot May 10, 2026
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LeetCode 364: Nested List Weight Sum II

Problem Restatement We are given a nested list. Each element is either: an integer or: a list containing integers or more nested lists We need to return the weighted sum of all integers. This problem uses inverse depth weights. That means integers closer to the top have larger weight, and integers at the deepest level have weight 1 . The official statement says this differs from the previous Nested List...
leetcode dfs bfs nested-list tree May 10, 2026
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LeetCode 109: Convert Sorted List to Binary Search Tree

Problem Restatement We are given the head of a singly linked list whose values are sorted in ascending order. We need to convert this sorted linked list into a height-balanced binary search tree. The official problem defines a height-balanced binary tree as a binary tree where the depth difference between the two subtrees of every node never exceeds 1 . The resulting tree must satisfy the BST property: left subtree...
leetcode linked-list tree binary-search-tree dfs divide-and-conquer two-pointers May 10, 2026
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LeetCode 211: Design Add and Search Words Data Structure

Problem Restatement We need to design a data structure that supports two operations: Operation Meaning addWord(word) Add a word into the data structure search(word) Return whether a word matches the query The search query may contain lowercase letters and the special character: . A dot matches any one lowercase letter. The official LeetCode problem asks us to implement WordDictionary with addWord and search , where search supports . as a...
leetcode trie dfs backtracking design May 10, 2026
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LeetCode 488: Zuma Game

Problem Restatement We are given a row of colored balls called board and some balls in our hand called hand . Each ball has one of five colors: Character Color R Red Y Yellow B Blue G Green W White On each move, we choose one ball from hand and insert it anywhere in board . After insertion, if there are three or more consecutive balls of the same color,...
leetcode string dfs memoization backtracking May 10, 2026
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LeetCode 108: Convert Sorted Array to Binary Search Tree

Problem Restatement We are given a sorted integer array nums in ascending order. We need to convert it into a height-balanced binary search tree. The official problem defines a height-balanced binary tree as a binary tree in which the depth difference between the two subtrees of every node is never more than 1 . The tree must also satisfy the binary search tree property: left subtree values < root value...
leetcode tree binary-search-tree dfs recursion divide-and-conquer May 10, 2026
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LeetCode 210: Course Schedule II

Problem Restatement We are given: numCourses prerequisites Each prerequisite pair: [a, b] means: to take course a, you must first complete course b We must return a valid order in which all courses can be completed. If it is impossible because of a cycle, return an empty list. The official statement says: return any valid ordering of courses that allows completion of all courses. If no ordering exists, return an...
leetcode graph topological-sort bfs dfs May 10, 2026
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LeetCode 236: Lowest Common Ancestor of a Binary Tree

Problem Restatement We are given: The root of a binary tree Two nodes p and q We need to return their lowest common ancestor. The lowest common ancestor is the lowest node in the tree that has both p and q as descendants. A node can be a descendant of itself. Unlike LeetCode 235, this tree is a normal binary tree, not a binary search tree. So we cannot use...
leetcode tree binary-tree dfs recursion May 10, 2026
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LeetCode 341: Flatten Nested List Iterator

Problem Restatement We are given a nested list of integers. Each element is either: Type Meaning Integer A single integer List A nested list whose elements may also be integers or lists We need to implement an iterator that returns the integers in flattened left-to-right order. The class must support: Method Meaning NestedIterator(nestedList) Initializes the iterator next() Returns the next integer hasNext() Returns true if there are more integers LeetCode...
leetcode stack dfs iterator design May 10, 2026
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LeetCode 437: Path Sum III

Problem Restatement We are given the root of a binary tree and an integer targetSum . We need to count how many paths have node values that add up to targetSum . A valid path: Rule Meaning Can start anywhere It does not have to start at the root Can end anywhere It does not have to end at a leaf Must go downward It can only move from parent...
leetcode tree binary-tree dfs prefix-sum hash-table May 10, 2026
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LeetCode 235: Lowest Common Ancestor of a Binary Search Tree

Problem Restatement We are given: The root of a binary search tree Two nodes p and q We need to find their lowest common ancestor. The lowest common ancestor, usually called LCA, is the lowest node in the tree such that both p and q are descendants of that node. A node may be a descendant of itself. LeetCode examples include: Input: root = [6,2,8,0,4,7,9,null,null,3,5] p = 2 q =...
leetcode tree binary-search-tree dfs May 10, 2026
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LeetCode 282: Expression Add Operators

Problem Restatement We are given: A string num containing only digits. An integer target . We need to insert the operators: +, -, * between the digits so that the final mathematical expression evaluates exactly to target . We must return all valid expressions. Digits must remain in the original order. We may also choose to join consecutive digits into a larger number. For example: "123" can become: "1+2+3" "12-3"...
leetcode backtracking dfs string recursion May 10, 2026
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LeetCode 261: Graph Valid Tree

Problem Restatement We are given: An integer n A list of undirected edges The graph has nodes labeled from 0 to n - 1 . Each edge: [a, b] means there is an undirected connection between node a and node b . We need to return True if these edges form a valid tree. Otherwise, return False . A valid tree must satisfy two properties: Property Meaning Connected Every node...
leetcode graph union-find dfs tree May 10, 2026
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LeetCode 337: House Robber III

Problem Restatement We are given the root of a binary tree. Each node represents a house, and the value of the node is the amount of money in that house. A thief wants to rob houses, but there is one rule: If two directly connected houses are robbed, the police are alerted. In tree terms, we cannot rob both a parent node and its child node. Return the maximum amount...
leetcode tree dfs dynamic-programming binary-tree May 10, 2026
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LeetCode 133: Clone Graph

Problem Restatement We are given a reference to a node in a connected undirected graph. Each node has: Field Meaning val Integer value of the node neighbors List of neighboring node references We need to return a deep copy of the graph. A deep copy means: Every original node gets a new node object. Every edge is copied between the new node objects. No cloned node should point back to...
leetcode Medium graph dfs bfs hash-map May 10, 2026
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LeetCode 207: Course Schedule

Problem Restatement We are given: numCourses a list of prerequisite pairs Each pair: [a, b] means: to take course a, you must first complete course b We need to determine whether it is possible to finish all courses. The official statement says: there are numCourses courses labeled from 0 to numCourses - 1 . You are given an array prerequisites where prerequisites[i] = [ai, bi] indicates that you must take...
leetcode graph topological-sort dfs bfs May 10, 2026
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LeetCode 106: Construct Binary Tree from Inorder and Postorder Traversal

Problem Restatement We are given two integer arrays: inorder postorder Both arrays describe the same binary tree. inorder is the inorder traversal of the tree. postorder is the postorder traversal of the same tree. We need to construct and return the original binary tree. The values are unique, and both arrays are guaranteed to describe the same tree. The official constraints also say each value in postorder appears in inorder...
leetcode tree binary-tree dfs recursion hash-map divide-and-conquer May 10, 2026
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LeetCode 105: Construct Binary Tree from Preorder and Inorder Traversal

Problem Restatement We are given two integer arrays: preorder inorder Both arrays describe the same binary tree. preorder is the preorder traversal of the tree. inorder is the inorder traversal of the same tree. We need to construct and return the original binary tree. The official problem states that preorder and inorder are traversals of the same tree. The key traversal rules are: preorder: root -> left -> right inorder:...
leetcode tree binary-tree dfs recursion hash-map May 10, 2026
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LeetCode 257: Binary Tree Paths

Problem Restatement We are given the root of a binary tree. We need to return all paths from the root to every leaf node. A leaf node is a node with no left child and no right child. Each path must be returned as a string using "->" between node values. For example, this path: 1 \ 2 \ 5 becomes: "1->2->5" The problem asks for all root-to-leaf paths in...
leetcode tree binary-tree dfs backtracking string May 10, 2026
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LeetCode 104: Maximum Depth of Binary Tree

Problem Restatement We are given the root of a binary tree. We need to return its maximum depth. The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node. The official statement uses this same definition. For this tree: 3 / \ 9 20 / \ 15 7 The longest root-to-leaf path is: 3 -> 20 -> 15 or:...
leetcode tree binary-tree dfs recursion May 10, 2026
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LeetCode 230: Kth Smallest Element in a BST

Problem Restatement We are given the root of a binary search tree and an integer k . We need to return the k th smallest value among all node values in the tree. The counting is 1-indexed. So: k = 1 means the smallest value k = 2 means the second smallest value k = 3 means the third smallest value LeetCode states that 1 <= k <= n <=...
leetcode tree binary-search-tree dfs stack May 10, 2026
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LeetCode 404: Sum of Left Leaves

Problem Restatement We are given the root of a binary tree. A leaf node is a node with no children: node.left is None and node.right is None A left leaf is a leaf node that is also the left child of its parent. We must return the sum of all left leaves in the tree. Input and Output Item Meaning Input The root of a binary tree Output The sum...
leetcode tree dfs binary-tree recursion May 10, 2026
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LeetCode 333: Largest BST Subtree

Problem Restatement We are given the root of a binary tree. We need to find the size of the largest subtree that is also a valid Binary Search Tree (BST). The size of a subtree means the number of nodes inside it. A BST follows these rules: Rule Meaning Left subtree All values are smaller than the root Right subtree All values are larger than the root Recursive property Both...
leetcode tree binary-search-tree dfs postorder May 10, 2026
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LeetCode 431: Encode N-ary Tree to Binary Tree

Problem Restatement We need to design a way to convert an N-ary tree into a binary tree and then reconstruct the original N-ary tree. Two methods are required: Method Meaning encode(root) Convert an N-ary tree into a binary tree decode(root) Convert the binary tree back into the original N-ary tree The conversion must preserve the full tree structure. An N-ary tree node can have any number of children. A binary...
leetcode tree n-ary-tree binary-tree dfs encoding May 10, 2026
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LeetCode 332: Reconstruct Itinerary

Problem Restatement We are given a list of airline tickets. Each ticket is a pair: [from, to] meaning there is a flight from airport from to airport to . We need to reconstruct the itinerary in order. Rules: Rule Meaning Start airport The itinerary must start from "JFK" Use all tickets Every ticket must be used exactly once Lexicographic order If multiple valid itineraries exist, return the smallest one Guarantee...
leetcode graph dfs eulerian-path hierholzer May 10, 2026
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LeetCode 156: Binary Tree Upside Down

Problem Restatement We are given the root of a binary tree. We need to flip the tree upside down and return the new root. The transformation follows these rules: The original left child becomes the new root of that local subtree. The original root becomes the new right child. The original right child becomes the new left child. The operation is done level by level. The input has an important...
leetcode binary-tree dfs recursion May 10, 2026
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LeetCode 130: Surrounded Regions

Problem Restatement We are given an m x n board containing only two characters: Character Meaning "X" Wall or blocking cell "O" Open cell We need to capture every region of "O" cells that is completely surrounded by "X" cells. A region is connected 4-directionally, meaning cells connect through up, down, left, and right. Diagonal connection does not count. To capture a surrounded region, we flip every "O" in that...
leetcode Medium array matrix dfs bfs union-find May 10, 2026
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LeetCode 430: Flatten a Multilevel Doubly Linked List

Problem Restatement We are given the head of a multilevel doubly linked list. Each node has four fields: Field Meaning val Node value prev Previous node on the same level next Next node on the same level child Head of another doubly linked list The child list can also contain nodes with their own child lists. We need to flatten everything into one single-level doubly linked list. When a node...
leetcode linked-list doubly-linked-list dfs recursion May 10, 2026
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LeetCode 129: Sum Root to Leaf Numbers

Problem Restatement We are given the root of a binary tree. Each node contains a digit from 0 to 9 . Every root-to-leaf path represents one number. For example, the path 1 -> 2 -> 3 represents the number 123 . We need to return the sum of all numbers formed by all root-to-leaf paths. A leaf is a node with no children. The answer is guaranteed to fit in...
leetcode Medium tree dfs binary-tree recursion May 10, 2026
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LeetCode 329: Longest Increasing Path in a Matrix

Problem Restatement We are given an m x n integer matrix. Return the length of the longest strictly increasing path in the matrix. From each cell, we may move in four directions: up, down, left, right We may not move diagonally, and we may not move outside the matrix boundary. The path must be strictly increasing, so every next cell must have a larger value than the current cell. Input...
leetcode matrix dfs dynamic-programming memoization May 10, 2026
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LeetCode 254: Factor Combinations

Problem Restatement We are given a positive integer n . We need to return all possible ways to write n as a product of factors. Each factor must be between 2 and n - 1 . For example: 8 = 2 * 2 * 2 8 = 2 * 4 So for n = 8 , valid answers are: [[2, 2, 2], [2, 4]] We may return the combinations in...
leetcode backtracking dfs math factorization May 10, 2026
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LeetCode 428: Serialize and Deserialize N-ary Tree

Problem Restatement We are given an N-ary tree. An N-ary tree is a rooted tree where each node can have zero or more children. We need to design two methods: Method Meaning serialize(root) Convert the tree into a string deserialize(data) Convert the string back into the original tree There is no required serialization format. We can choose any format as long as it can rebuild the same tree structure. The...
leetcode tree n-ary-tree dfs serialization recursion May 10, 2026
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LeetCode 403: Frog Jump

Problem Restatement A frog wants to cross a river by jumping on stones. The stones are given as a sorted array: stones Each value in stones is a position on the river. The frog starts on the first stone, which is always position 0 . The first jump must be exactly 1 unit. After that, if the frog’s previous jump was k units, then the next jump must be one...
leetcode dynamic-programming hash-set dfs memoization May 10, 2026
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LeetCode 426: Convert Binary Search Tree to Sorted Doubly Linked List

Problem Restatement We are given the root of a binary search tree. We need to convert the tree into a sorted circular doubly linked list in-place. “In-place” means we should reuse the existing tree nodes. We should not create new list nodes. After conversion: Tree Pointer List Meaning left previous node right next node The returned node should be the smallest node in the list. The list must also be...
leetcode binary-search-tree tree dfs linked-list inorder-traversal May 10, 2026
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LeetCode 351: Android Unlock Patterns

Problem Restatement We have a 3 x 3 Android lock screen. The dots are numbered like this: 1 2 3 4 5 6 7 8 9 Given two integers m and n , count how many valid unlock patterns have length at least m and at most n . A pattern is a sequence of distinct dots. A move from one dot to another is valid if either: The line...
leetcode backtracking dfs matrix android-unlock-patterns May 10, 2026
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LeetCode 226: Invert Binary Tree

Problem Restatement We are given the root of a binary tree. We need to invert the tree and return its root. Inverting means every node swaps its left child and right child. For example, LeetCode gives this input and output: root = [4,2,7,1,3,6,9] becomes [4,7,2,9,6,3,1] . The constraints say the tree has between 0 and 100 nodes, and each node value is between -100 and 100 . Input and Output...
leetcode binary-tree dfs recursion May 10, 2026
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LeetCode 52: N-Queens II

Problem Restatement We are given an integer n . We need to count how many ways we can place n queens on an n x n chessboard so that no two queens attack each other. A queen can attack another queen if they are in the same row, same column, or same diagonal. Unlike LeetCode 51, we do not need to return the boards. We only return the number of...
leetcode backtracking dfs n-queens May 10, 2026
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LeetCode 101: Symmetric Tree

Problem Restatement We are given the root of a binary tree. We need to check whether the tree is symmetric around its center. In other words, the left subtree must be a mirror image of the right subtree. The official problem asks whether the binary tree is a mirror of itself. A tree like this is symmetric: 1 / \ 2 2 / \ / \ 3 4 4 3...
leetcode tree binary-tree dfs recursion May 10, 2026
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LeetCode 51: N-Queens

Problem Restatement We are given an integer n . We need to place n queens on an n x n chessboard so that no two queens attack each other. A queen can attack another queen if they share: The same row The same column The same diagonal Return all distinct valid board configurations. Each board uses "Q" for a queen and "." for an empty square. The answer may be...
leetcode backtracking array dfs n-queens May 10, 2026