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### leetcode Trapping Rain Water

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.

For example, Given [0,1,0,2,1,0,1,3,2,1,2,1], return 6.

The above elevation map is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped. Thanks Marcos for contributing this image!

### leetcode Rotate Image

You are given an n x n 2D matrix representing an image. Rotate the image by 90 degrees (clockwise). Follow up: Could you do this in-place?

Given an unsorted integer array, find the first missing positive integer.

For example, Given [1,2,0] return 3, and [3,4,-1,1] return 2.

Your algorithm should run in O(n) time and uses constant space.

• 31 Next Permutation
• 46 Permutations
• 47 Permutations II
• 60 Permutation Sequence

• 45. Jump Game II
• 55. Jump Game

The count-and-say sequence is the sequence of integers beginning as follows: 1, 11, 21, 1211, 111221, ...

1 is read off as "one 1" or 11. 11 is read off as "two 1s" or 21. 21 is read off as "one 2, then one 1" or 1211.

Given an integer n, generate the _n_th sequence.

Note: The sequence of integers will be represented as a string.

• 36. Valid Sudoku
• 37. Sudoku Solver

Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.

Example 1:

Input: "(()" Output: 2 Explanation: The longest valid parentheses substring is "()"

Example 2:

Input: ")()())" Output: 4 Explanation: The longest valid parentheses substring is "()()"

You are given a string, S, and a list of words, L, that are all of the same length. Find all starting indices of substring(s) in S that is a concatenation of each word in L exactly once and without any intervening characters.

For example, given: S: "barfoothefoobarman" L: ["foo", "bar"]

You should return the indices: [0,9]. (order does not matter).

Divide two integers without using multiplication, division and mod operator.

If it is overflow, return MAX_INT.