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#iterator

16 pages tagged

Practice › LeetCode › LeetCode 02xx ›

LeetCode 281 - Zigzag Iterator

LeetCode Problem 281 Difficulty: 🟡 Medium Topics: Array, Design, Queue, Iterator Solution Problem Understanding The problem asks us to design an iterator that alternates between two input arrays, returning elements in a cyclic or zigzag order. Instead of consuming all elements from the first vector and then moving to the second vector, we must switch back and forth between them whenever possible. For example, if the input vectors are [1,...
leetcode Medium array design queue iterator May 28, 2026
Practice › LeetCode › LeetCode 02xx ›

LeetCode 251 - Flatten 2D Vector

LeetCode Problem 251 Difficulty: 🟡 Medium Topics: Array, Two Pointers, Design, Iterator Solution Problem Understanding The problem asks us to design an iterator that traverses a two dimensional array as though it were a single flat sequence. Instead of returning nested lists, the iterator should expose elements one at a time through two methods, next() and hasNext() . The input is a 2D vector, meaning a list of lists. Each...
leetcode Medium array two-pointers design iterator May 28, 2026
Practice › LeetCode › LeetCode 15xx ›

LeetCode 1586 - Binary Search Tree Iterator II

LeetCode Problem 1586 Difficulty: 🟡 Medium Topics: Stack, Tree, Design, Binary Search Tree, Binary Tree, Iterator Solution Problem Understanding The problem requires implementing a BSTIterator class that allows forward and backward traversal over the in-order sequence of a Binary Search Tree (BST). In-order traversal of a BST visits nodes in ascending order. The iterator must support four operations: next() , prev() , hasNext() , and hasPrev() . Initially, the iterator...
leetcode Medium stack tree design binary-search-tree binary-tree iterator May 28, 2026
Practice › LeetCode › LeetCode 01xx ›

LeetCode 173 - Binary Search Tree Iterator

LeetCode Problem 173 Difficulty: 🟡 Medium Topics: Stack, Tree, Design, Binary Search Tree, Binary Tree, Iterator Solution It looks like your prompt accidentally concatenated two problems. I will answer the second one, LeetCode 173 - Binary Search Tree Iterator , following your required format. Problem Understanding This problem asks us to design an iterator for a Binary Search Tree (BST) that returns values in in-order traversal order . In a...
leetcode Medium stack tree design binary-search-tree binary-tree iterator May 28, 2026
Practice › LeetCode › LeetCode 02xx ›

LeetCode 284 - Peeking Iterator

LeetCode Problem 284 Difficulty: 🟡 Medium Topics: Array, Design, Iterator Solution Problem Understanding The problem asks us to extend the functionality of a standard iterator with an additional peek operation. A standard iterator provides two operations: next() , which returns the next element in the sequence and advances the pointer, and hasNext() , which tells whether there are more elements left. The PeekingIterator enhances this interface with peek() , which...
leetcode Medium array design iterator May 28, 2026
Practice › LeetCode › LeetCode 09xx ›

LeetCode 900 - RLE Iterator

LeetCode Problem 900 Difficulty: 🟡 Medium Topics: Array, Design, Counting, Iterator Solution Problem Understanding This problem asks us to design an iterator over a run-length encoded sequence instead of storing the full sequence explicitly. In run-length encoding, the array is organized in pairs: encoding[i] represents how many times a value appears encoding[i + 1] represents the value itself For example: [3,8,2,5] means: 8 appears 3 times 5 appears 2 times...
leetcode Medium array design counting iterator May 28, 2026
Practice › LeetCode › LeetCode 06xx ›

LeetCode 604 - Design Compressed String Iterator

LeetCode Problem 604 Difficulty: 🟢 Easy Topics: Array, String, Design, Iterator Solution Problem Understanding This problem asks us to implement a compressed string iterator . We are given a string where each character is immediately followed by a number representing how many times that character appears consecutively in the uncompressed version. For example, "a3b2" represents "aaabb" . We are required to implement a class StringIterator with two methods: next() ,...
leetcode Easy array string design iterator May 28, 2026
Practice › LeetCode › LeetCode 03xx ›

LeetCode 341 - Flatten Nested List Iterator

LeetCode Problem 341 Difficulty: 🟡 Medium Topics: Stack, Tree, Depth-First Search, Design, Queue, Iterator Solution Problem Understanding This problem asks us to design an iterator that can traverse a deeply nested list structure as if it were a flat list of integers. The input is not a normal array. Each element inside nestedList can be one of two things: A single integer Another list containing more NestedInteger objects Those nested...
leetcode Medium stack tree depth-first-search design queue iterator May 28, 2026
Practice › LeetCode › LeetCode 12xx ›

LeetCode 1286 - Iterator for Combination

LeetCode Problem 1286 Difficulty: 🟡 Medium Topics: String, Backtracking, Design, Iterator Solution Problem Understanding The problem asks us to design a class CombinationIterator that generates all combinations of a given length from a string of sorted, distinct lowercase letters. The iterator must return these combinations in lexicographical order , meaning alphabetically. The input consists of two components: a string characters , which contains unique letters sorted in ascending order, and...
leetcode Medium string backtracking design iterator May 28, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 09xx ›

LeetCode 900: RLE Iterator

Problem Restatement We need to design an iterator over a run-length encoded sequence. The encoded array stores pairs: [count, value] At every even index i , encoding[i] is the number of times encoding[i + 1] appears in the decoded sequence. For example: encoding = [3, 8, 0, 9, 2, 5] represents: [8, 8, 8, 5, 5] because there are three 8 s, zero 9 s, and two 5 s. The...
leetcode array design iterator simulation May 12, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 06xx ›

LeetCode 604: Design Compressed String Iterator

Problem Restatement We need to design a class called StringIterator . The input is a compressed string. The compressed string has repeated groups of: letter + positive integer The integer tells us how many times that letter appears in the original uncompressed string. For example: "L1e2t1C1o1d1e1" represents: "LeetCode" because: Compressed Part Meaning L1 L appears once e2 e appears twice t1 t appears once C1 C appears once o1 o...
leetcode design string iterator array May 11, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 173: Binary Search Tree Iterator

Problem Restatement We need to implement a class: BSTIterator It represents an iterator over the inorder traversal of a binary search tree. For a BST, inorder traversal visits values in ascending order. The class must support: Method Meaning BSTIterator(root) Initialize the iterator with the root of the BST next() Move to the next value and return it hasNext() Return whether there is still a next value The pointer starts before...
leetcode stack tree design binary-search-tree iterator May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 284: Peeking Iterator

Problem Restatement We are given an existing iterator with these operations: next() hasNext() We need to design a new iterator called PeekingIterator that also supports: peek() peek() should return the next element without moving the iterator forward. That means calling: peek() multiple times should always return the same value until: next() is called. The official statement defines peek() as retrieving the next element without advancing the iterator. The wrapped iterator...
leetcode design iterator object-oriented-programming May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

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
Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 281: Zigzag Iterator

Problem Restatement We are given two integer lists, v1 and v2 . We need to implement an iterator that returns their elements alternately. For example: v1 = [1, 2] v2 = [3, 4, 5, 6] The iterator should return: 1, 3, 2, 4, 5, 6 Once one list runs out, we continue returning elements from the other list. The class has three operations: ZigzagIterator(v1, v2) next() hasNext() The source statement...
leetcode design queue iterator array May 10, 2026
Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 251: Flatten 2D Vector

Problem Restatement We need to design an iterator over a 2D vector. The input is a list of lists, for example: vec = [[1, 2], [3], [4]] The iterator should behave as if the numbers were stored in one flat list: [1, 2, 3, 4] The class must support two operations: Method Meaning next() Return the next integer and move the iterator forward hasNext() Return True if another integer exists,...
leetcode design array iterator two-pointers May 10, 2026