Learnitweb

Author: Editorial Team

  • Concatenation of Array

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    1. Problem Summary You are given an integer array nums of length n. Your task is to build a new array ans of length 2n such that: for every index i from 0 to n−1. In other words, you must concatenate the array with itself: Example: 2. Key Insights Insight 1: Output size always exactly…

  • Build Array from Permutation

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    1. Problem Summary You are given an integer array nums of length n, where: You must construct a new array ans such that: and return ans. Example: 2. Key Insights Insight 1: Nested indexing is always valid Since every value in nums is a valid index, accessing nums[nums[i]] is always safe. Insight 2: No reordering…

  • Build Array from Permutation

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    1. Problem Summary You are given an integer array nums where: Your task is to construct a new array ans such that: Then return ans. Example: 2. Key Insights Insight 1: Direct indexing solves it Since nums is a permutation, every index lookup is valid. Insight 2: No need for additional logic No sorting, mapping,…

  • LeetCode Problem 126 Word Ladder II

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    1. Problem Summary You are given: Your task is to: Return all shortest transformation sequences from beginWord to endWord, such that: Example: Input: Output: 2. Key Insights Insight 1: This is a shortest-path problem in word graph Each word is a node.Edges exist between words that differ by exactly one letter. Insight 2: BFS ensures…

  • Binary Tree Pruning

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    1. Problem Summary You are given the root of a binary tree where each node has a value of: Your task is to: Remove (prune) every subtree that does not contain at least one node with value 1. Rules and clarifications: Example: Input: After pruning, output: Another example: Input: After pruning, output: 2. Key Insights…

  • Partition Array into Disjoint Intervals

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    1. Problem Summary You are given an integer array nums. Your task is to partition the array into two contiguous parts: such that: You must return the smallest possible size of left, meaning the smallest index i + 1. Example: 2. Key Insights Insight 1: Left must contain all values up to its max For…

  • Longest Consecutive Sequence

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    1. Problem Summary You are given an unsorted integer array nums. Your task is to find the length of the longest consecutive sequence, where: Example: 2. Key Insights Insight 1: Sorting works, but too slow Sorting takes: But we need an O(N) solution. Insight 2: HashSet enables O(1) lookups If values are stored in a…

  • Max Area of Island

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    1. Problem Summary You are given a 2D grid consisting of: An island is defined as a group of connected 1s where connectivity is: Your task is to compute: If there is no land at all, return: Example: 2. Key Insights Insight 1: This is a connected-component counting problem Each island is a connected region…

  • Maximum Product of Word Lengths

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    1. Problem Summary You are given an array of strings words. Your task is to find the maximum value of: where: Example: Valid pair: Lengths: Output: 2. Key Insights Insight 1: Checking shared letters directly is too slow Naive comparison of characters between every pair is: Where L = max word length. Insight 2: Letters…

  • Partitioning Into Minimum Number of Deci-Binary Numbers

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    1. Problem Summary You are given a string n representing a positive decimal integer.Your task is to determine the minimum number of positive deci-binary numbers whose sum equals n. A deci-binary number is defined as: Goal: Example: 2. Key Insights Insight 1: Digits add independently by position Since deci-binary numbers only contain 0 or 1…