To solve LeetCode Permutation Sequence, math could be adopted to identify the correct number in the corresponding position rather than do DFS simulation.

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LeetCode Next Permutation: Find the math rule of how to get the next permutation (the smallest larger one) which involves swap and reverse to tackle this problem.

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LeetCode Maximal Rectangle: 1) Dynamic Programming with O(m * n * m) time; 2) Use Largest Rectangle in Histogram (Increasing Stack) to get O(m * n) approach.

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leetcode: Search for a Range solution

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leetcode: Merge Intervals solution

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