Trapping rain water@leetcode

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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!

» Solve this problem

For this problem, the key point is to know how to use the number in the array to calculate the water trap.

We notice that, the water we can collect depends on the min(leftHeight[i], rightHeight[i])-A[i]. leftHeight[i] is the highest left bar of A[i]. So we can use two array to get the leftHeight and rightHeight. if min(leftHeight[i], rightHeight[i])-A[i] < 0, then res[i]=0. After we get the res[], we can just simply add every elements up.