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LeetCode 773: Sliding Puzzle

Problem Restatement We are given a 2 x 3 sliding puzzle board. The board contains the numbers 1 through 5 and one empty slot represented by 0 . A move consists of swapping 0 with one of its 4-directionally adjacent numbers. The goal state is: [[1, 2, 3], [4, 5, 0]] Return the least number of moves needed to reach the goal state. If it is impossible, return -1 ....

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LeetCode 818: Race Car

Problem Restatement We have a race car on an infinite number line. The car starts at: position = 0 speed = 1 There are two possible instructions. Instruction Effect "A" position += speed , then speed *= 2 "R" If speed is positive, set speed to -1 ; otherwise set speed to 1 The "R" instruction changes only the direction and resets speed magnitude to 1 . It does not...

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LeetCode 815: Bus Routes

Problem Restatement We are given a list of bus routes. Each routes[i] is the route of bus i . The bus repeats this route forever. For example: routes[0] = [1, 5, 7] means bus 0 travels like this forever: 1 -> 5 -> 7 -> 1 -> 5 -> 7 -> ... We start at bus stop source . We are not on any bus initially. We want to reach...

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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...

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LeetCode 864: Shortest Path to Get All Keys

Problem Restatement We are given a 2D grid. Each cell contains one character: Character Meaning "." Empty cell "#" Wall "@" Starting position "a" to "f" Key "A" to "F" Lock We start at "@" . One move means walking one cell up, down, left, or right. We cannot walk outside the grid. We cannot walk through walls. When we step on a key, we pick it up. We cannot...

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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...

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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,...

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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...

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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...

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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...

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LeetCode 854: K-Similar Strings

Problem Restatement We are given two strings s1 and s2 . The two strings are anagrams. That means they contain the same letters with the same frequencies. One operation lets us swap any two characters in s1 . We need to return the smallest number of swaps needed to turn s1 into s2 . This smallest number is called k . For example: s1 = "abc" s2 = "bca" One...

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LeetCode 515: Find Largest Value in Each Tree Row

Problem Restatement We are given the root of a binary tree. For every depth level in the tree, we need to find the largest node value on that row. Return the results as an array where: answer[i] is the maximum value on depth i . The official problem asks us to return the largest value in each row of a binary tree. Input and Output Item Meaning Input The root...

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LeetCode 513: Find Bottom Left Tree Value

Problem Restatement We are given the root of a binary tree. We need to return the leftmost value in the last row of the tree. The last row means the deepest level of the tree. The leftmost value means the first node on that deepest level when reading from left to right. The official problem asks us to return the leftmost value in the last row of a binary tree....

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LeetCode 505: The Maze II

Problem Restatement We are given a maze represented as a grid. Value Meaning 0 Empty space 1 Wall A ball starts at position start . The goal is to reach position destination . The ball does not move one cell at a time. Instead, once the ball starts moving in a direction, it keeps rolling until it hits a wall. Only then can it choose another direction. We need to...

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LeetCode 752: Open the Lock

Problem Restatement We have a lock with four wheels. Each wheel contains digits from 0 to 9 . The lock starts at: "0000" In one move, we can rotate exactly one wheel by one step. The digits wrap around: 9 -> 0 0 -> 9 Some lock states are deadends. If the lock reaches one of those states, it gets stuck and cannot move anymore. Given a list of deadends...

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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]...

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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...

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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...

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LeetCode 444: Sequence Reconstruction

Problem Restatement We are given: Name Meaning nums The target sequence sequences A list of subsequences The array nums is a permutation of integers from 1 to n . Each sequence in sequences is a subsequence of nums . We need to check whether nums is the only shortest supersequence that contains every sequence in sequences as a subsequence. A supersequence means it contains another sequence in the same relative...

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LeetCode 317: Shortest Distance from All Buildings

Problem Restatement We are given an m x n grid. Each cell has one of three values: Value Meaning 0 Empty land 1 Building 2 Obstacle We want to build a house on an empty land cell. The house must be able to reach every building by moving only up, down, left, and right. Buildings and obstacles cannot be passed through. Return the minimum total travel distance from one empty...

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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...

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LeetCode 269: Alien Dictionary

Problem Restatement We are given a list of words sorted according to an unknown alien alphabet. We need to infer one possible order of the alien alphabet. Return a string containing all unique letters in a valid order. If no valid order exists, return: "" If multiple valid orders exist, any one of them is accepted. The constraints are 1 <= words.length <= 100 , 1 <= words[i].length <= 100...

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LeetCode 117: Populating Next Right Pointers in Each Node II

Problem Restatement We are given the root of a binary tree. Each node has four fields: val left right next We must populate every next pointer so that it points to the next node on the same level. If there is no node to the right, the next pointer should be None . Unlike LeetCode 116, the tree is not necessarily perfect. Some nodes may have only one child or...

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LeetCode 116: Populating Next Right Pointers in Each Node

Problem Restatement We are given a perfect binary tree. A perfect binary tree has two important properties: Every internal node has exactly two children. All leaves are on the same level. Each node has four fields: val left right next We need to populate each next pointer so that it points to the node immediately to its right on the same level. If there is no node to the right,...

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LeetCode 111: Minimum Depth of Binary Tree

Problem Restatement We are given the root of a binary tree. We need to return its minimum depth. The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node. A leaf is a node with no children. For this tree: 3 / \ 9 20 / \ 15 7 The nearest leaf is 9 . The path is: 3...

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LeetCode 314: Binary Tree Vertical Order Traversal

Problem Restatement We are given the root of a binary tree. Return the vertical order traversal of the tree's node values. Vertical order means: Group nodes by column. Output columns from left to right. Inside each column, output nodes from top to bottom. If two nodes have the same row and column, output the left node before the right node. The root starts at column 0 . For any node:...

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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...

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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...

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LeetCode 107: Binary Tree Level Order Traversal II

Problem Restatement We are given the root of a binary tree. We need to return the node values level by level, but from bottom to top. Inside each level, values must still be ordered from left to right. The official problem asks for the bottom-up level order traversal of the binary tree's node values, from leaf level to root level. For this tree: 3 / \ 9 20 / \...

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LeetCode 310: Minimum Height Trees

Problem Restatement We are given a tree with n nodes labeled from 0 to n - 1 . The tree is given as an undirected edge list edges , where: edges[i] = [a, b] means node a and node b are connected. We may choose any node as the root. The height of the rooted tree is the number of edges on the longest downward path from the root to...

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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...

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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...

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LeetCode 407: Trapping Rain Water II

Problem Restatement We are given an m x n matrix called heightMap . Each cell represents the height of one unit block in a 2D elevation map. After rain, water may be trapped in lower cells if surrounding boundaries prevent it from flowing out. Return the total amount of trapped water. Water can move through the four main directions: up, down, left, right Diagonal movement does not matter. Boundary cells...

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LeetCode 433: Minimum Genetic Mutation

Problem Restatement We are given two gene strings: Name Meaning startGene The starting gene endGene The target gene Each gene string has length 8 and contains only these characters: "A", "C", "G", "T" One mutation means changing exactly one character. For example: AACCGGTT -> AACCGGTA This is one mutation because only the last character changed. We are also given a gene bank. A gene is valid only if it appears...

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LeetCode 103: Binary Tree Zigzag Level Order Traversal

Problem Restatement We are given the root of a binary tree. We need to return the values level by level. The first level is read from left to right, the second level from right to left, the third level from left to right, and so on. The official problem asks for the zigzag level order traversal of a binary tree's node values. For this tree: 3 / \ 9 20...

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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...

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LeetCode 429: N-ary Tree Level Order Traversal

Problem Restatement We are given the root of an N-ary tree. We need to return the level order traversal of its node values. Level order traversal means: Visit all nodes on level 0 Then all nodes on level 1 Then all nodes on level 2 Continue until the tree is finished For each level, we store the node values in a separate list. The final answer is therefore a list...

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LeetCode 102: Binary Tree Level Order Traversal

Problem Restatement We are given the root of a binary tree. We need to return the node values level by level, from left to right inside each level. The official problem asks for the level order traversal of the binary tree's node values. For this tree: 3 / \ 9 20 / \ 15 7 The answer is: [[3], [9, 20], [15, 7]] Each inner list contains one tree level....

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LeetCode 127: Word Ladder

Problem Restatement We are given: Name Meaning beginWord Starting word endWord Target word wordList Dictionary of allowed words We may transform one word into another by changing exactly one character. Every transformed word must exist in wordList . We need to return the number of words in the shortest transformation sequence from beginWord to endWord . If no valid sequence exists, return 0 . Examples Example 1: beginWord = "hit"...

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LeetCode 301: Remove Invalid Parentheses

Problem Restatement We are given a string s containing parentheses and letters. We need to remove the minimum number of invalid parentheses so that the remaining string becomes valid. Return all unique valid strings that can be produced with the minimum number of removals. The answer may be returned in any order. A valid parentheses string means: Every ( has a matching ) . Every ) matches some earlier (...

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LeetCode 126: Word Ladder II

Problem Restatement We are given three things: Name Meaning beginWord The word where the transformation starts endWord The word we want to reach wordList The dictionary of valid transformed words We need to return all shortest transformation sequences from beginWord to endWord . A valid transformation sequence looks like this: beginWord -> s1 -> s2 -> ... -> endWord Each step must obey two rules: Adjacent words must differ by...

#bfs

LeetCode 773: Sliding Puzzle

LeetCode 818: Race Car

LeetCode 815: Bus Routes

LeetCode 623: Add One Row to Tree

LeetCode 864: Shortest Path to Get All Keys

LeetCode 863: All Nodes Distance K in Binary Tree

LeetCode 886: Possible Bipartition

LeetCode 695: Max Area of Island

LeetCode 785: Is Graph Bipartite?

LeetCode 690: Employee Importance

LeetCode 854: K-Similar Strings

LeetCode 515: Find Largest Value in Each Tree Row

LeetCode 513: Find Bottom Left Tree Value

LeetCode 505: The Maze II

LeetCode 752: Open the Lock

LeetCode 582: Kill Process

LeetCode 323: Number of Connected Components in an Undirected Graph

LeetCode 417: Pacific Atlantic Water Flow

LeetCode 444: Sequence Reconstruction

LeetCode 317: Shortest Distance from All Buildings

LeetCode 490: The Maze

LeetCode 269: Alien Dictionary

LeetCode 117: Populating Next Right Pointers in Each Node II

LeetCode 116: Populating Next Right Pointers in Each Node

LeetCode 111: Minimum Depth of Binary Tree

LeetCode 314: Binary Tree Vertical Order Traversal

LeetCode 364: Nested List Weight Sum II

LeetCode 210: Course Schedule II

LeetCode 107: Binary Tree Level Order Traversal II

LeetCode 310: Minimum Height Trees

LeetCode 133: Clone Graph

LeetCode 207: Course Schedule

LeetCode 407: Trapping Rain Water II

LeetCode 433: Minimum Genetic Mutation

LeetCode 103: Binary Tree Zigzag Level Order Traversal

LeetCode 130: Surrounded Regions

LeetCode 429: N-ary Tree Level Order Traversal

LeetCode 102: Binary Tree Level Order Traversal

LeetCode 127: Word Ladder

LeetCode 301: Remove Invalid Parentheses

LeetCode 126: Word Ladder II