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MEDIUMArray, BacktrackingThe input gives a set of distinct candidate numbers and a target. The task is to return every unique combination of candidates whose sum is exactly the target.
Two details define the real problem:
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MEDIUMStringThe problem gives a string and a row count, then asks for the string that would appear if the characters were written in a zigzag pattern and read row by row.
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MEDIUMMath, Dynamic Programming, CombinatoricsThere is an m x n grid. A robot starts in the top-left corner and wants to reach the bottom-right corner. The robot can only move right or down.
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MEDIUMArray, Binary Search, MatrixThe matrix looks two-dimensional, but the key rule makes it behave like one long sorted array:
So the task is not really “search a grid.” It is “search a sorted sequence” while remembering that the sequence happens to be stored in matrix form.
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MEDIUMArray, Hash Table, Sliding WindowThe story sounds specific, but the core problem is simple:
find the length of the longest contiguous subarray that contains at most two distinct values.
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HARDLinked List, Divide and Conquer, Heap (Priority Queue), Merge SortEach input list is already sorted. The task is to combine all of them into one final sorted linked list without losing that order.
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MEDIUMHash Table, String, BacktrackingEach digit from 2 to 9 maps to a small set of letters, just like an old phone keypad. Given a string of digits, the task is to return every possible letter string that could be formed by choosing one mapped letter for each digit.
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MEDIUMArray, Two Pointers, SortingThe surface task is to return every unique quadruplet whose sum equals target.
The harder part is not the arithmetic. It is avoiding duplicate answers while still searching efficiently.
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MEDIUMArray, BacktrackingThe input contains distinct integers, and the task is to return every possible ordering of those integers.
That means the algorithm is not trying to find one best answer. It is trying to systematically explore a decision tree where:
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MEDIUMArray, Math, MatrixThe input is an n x n matrix, and the task is to rotate it 90 degrees clockwise.
The important constraint is that the rotation must happen in place. That means the matrix itself must be rearranged without allocating another full n x n matrix for the final answer.