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. 

Solution:

 Given: 0 1 0 2 1 0 1 3 2 1 2 1
First traverse from left to right and get the maximum water height
Get A: 0 1 1 2 2 2 2 3 3 3 3 3
Then traverse from right to left to get the maximum water height on the other side
Get B: 3 3 3 3 3 3 3 3 2 2 2 1
Compare A and B to get the minimum, which is maximum allowable water height on each bar
Min(A, B): 0 1 1 2 2 2 2 3 2 2 2 1
Extract the height of each bar to get the allowable water level
Water: 0 0 1 0 1 2 1 0 0 1 0 0

The sum is 6.

The following codes pass LeetCode Online Large Judge.

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