Generated by Codex with GPT-5
HARDArray, Two Pointers, Dynamic Programming, StackThe input is an elevation map where each number is the height of a vertical bar with width 1.
After rain falls, water can sit in the valleys between taller bars. The task is to compute the total amount of water that stays trapped.
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MEDIUMArray, Two Pointers, GreedyThe input is an array of heights. Each height represents a vertical line drawn at that index.
The task is to pick two lines that, together with the x-axis, form a container that holds the most water.
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MEDIUMLinked List, Math, RecursionTwo linked lists represent two non-negative integers. Each node stores one digit, and the digits are already in reverse order, so the ones place comes first.
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MEDIUMHash Table, String, Sliding WindowGiven a string, the goal is to find the length of the longest contiguous block of characters with no repeats.
The important word is contiguous. This is not asking for a subsequence that can skip characters. It is asking for one unbroken window inside the string.
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MEDIUM64.1Two Pointers, String, Dynamic ProgrammingGiven a string, return the longest contiguous substring that reads the same forward and backward.
The word “substring” is the important constraint. This is not about picking scattered characters. The answer has to be one continuous block inside the original string.
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MEDIUMArray, Hash Table, Prefix SumThe input is an integer array and a target k.
The task is not to find the longest subarray or to return the subarray itself. It is simply to count how many contiguous subarrays add up to k.