Generated by Codex with GPT-5
HARDString, Dynamic Programming, StackThe input is a string made only of ( and ).
The task is not to check whether the whole string is valid. The task is to find the length of the longest contiguous substring that forms a valid parentheses expression.
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HARDArray, Hash TableGiven an unsorted integer array, return the smallest positive integer that does not appear in the array.
The tricky part is not the definition of the answer. The tricky part is the constraint: the best solution must run in O(n) time and use only O(1) extra space.
Generated by Codex with GPT-5
MEDIUMHash Table, Math, StringThe input is an integer between 1 and 3999, and the output must be its Roman numeral form.
The main challenge is not basic lookup. The hard part is handling the subtractive cases correctly:
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MEDIUMArray, Depth-First Search, Breadth-First Search, Union-Find, MatrixThe board contains X and O. The task is to flip only the O cells that belong to regions fully enclosed by X.
The important phrase is “surrounded region.” A region is safe if it can reach the border through adjacent O cells. A region is captured only if there is no such path.
Generated by Codex with GPT-5
MEDIUMArray, Two Pointers, SortingThe input is an integer array and a target value. The job is to pick exactly three numbers whose sum is as close to the target as possible.
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MEDIUMDatabase, SQLThere is an Employee table with a salary column. The task is to return the second-highest distinct salary value.
That word “distinct” is the whole trick.
If the largest salary appears many times, it still counts as only one rank. So this is not:
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MEDIUMLinked List, Two PointersThe input is a singly linked list and a positive integer n.
The task is to delete the node that is n positions from the end and return the new head.
Generated by Codex with GPT-5
HARDString, Dynamic Programming, RecursionThis problem is not asking for a full regex engine.
It asks for one very specific kind of full-string match:
. matches exactly one arbitrary character* means “repeat the immediately previous token zero or more times”That last rule is the detail that makes many first attempts fail. A match is not enough if there are leftover characters in either the text or the pattern.
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HARDArray, Hash Table, Backtracking, MatrixThis problem is a constraint satisfaction problem disguised as a puzzle.
The board is almost complete already. The task is to fill each . with a digit so that three rules are always true:
Generated by Codex with GPT-5
MEDIUMArray, Binary Search, Dynamic ProgrammingAn array of integers is given. The task is to find the length of the longest subsequence whose values are strictly increasing.
The key word is subsequence, not subarray.