Generated by Codex with GPT-5
HARDHash Table, String, Sliding WindowThere are two strings:
s, the big string where the answer must come fromt, the set of required characters the answer must containThe task is to return the shortest substring of s that contains every character from t, including duplicates.
Generated by Codex with GPT-5
MEDIUMLinked ListThis problem sounds impossible the first time it is heard.
In a normal singly linked list, deleting a node means changing the previous node’s next pointer so it skips the node being removed. But this problem does not give the head of the list or the previous node. It gives only the node that should disappear, and it guarantees that this node is not the tail.
Generated by Codex with GPT-5
MEDIUMArray, Depth-First Search, Breadth-First Search, MatrixEach cell in the grid is a height. Water can flow from a cell to one of its four neighbors if the neighbor is at the same height or lower.
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MEDIUMArray, Dynamic Programming, Backtracking22.1Each number in nums must receive either a plus sign or a minus sign.
After choosing a sign for every element, the final expression must evaluate to target.
Generated by Codex with GPT-5
MEDIUMLinked List, Two Pointers25.6The input is a singly linked list and a non-negative integer k.
Rotating the list to the right by k means taking the last k nodes, preserving their order, and moving them to the front.
Generated by Codex with GPT-5
MEDIUMStringThe input is a string, and the task is to extract the integer prefix that follows a very specific parsing rule:
+ or -So this is not a math problem. It is a parsing problem with a few edge cases that must be handled in the correct order.
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MEDIUMArray, Backtracking, Bit ManipulationThis is the power set problem with one extra constraint that changes everything: the input can contain duplicate values, but the output cannot contain duplicate subsets.
If the array were all distinct, the job would be straightforward. At each position, either include the number or skip it, and every path through that decision tree would produce a unique subset.
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MEDIUMArray, Dynamic Programming, Stack, Monotonic Stack22.1For every contiguous subarray, take its minimum value. Then add all of those minimums together.
That sounds simple until the number of subarrays is considered. An array of length n has about n^2 / 2 subarrays, so listing each one and recomputing its minimum quickly becomes too slow.
Generated by Codex with GPT-5
HARDDynamic Programming, Tree, Depth-First Search, Binary TreeThis problem is easy to misread at first.
The path does not have to start at the root, and it does not have to end at a leaf. It can start anywhere and end anywhere, as long as it follows parent-child edges and does not reuse a node.
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HARDString, Dynamic Programming, GreedyThe pattern contains two special characters:
? matches exactly one character* matches any sequence of characters, including the empty sequenceThe match must cover the entire string. That is the part that makes the problem interesting.
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