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!

class Solution {public:    int trap(int A[], int n) {        int maxi = 0;        for(int ii = 1; ii < n; ii ++) {            if(A[maxi] < A[ii]) {                maxi = ii;            }        }                int result = 0;        // Left to max        int maxval = A[0];        for(int ii = 1; ii < maxi; ii ++) {            if(A[ii] > maxval) {                maxval = A[ii];            }            else {                result += maxval - A[ii];            }        }                // Right to max        maxval = A[n - 1];        for(int ii = n -2; ii > maxi; ii --) {            if(A[ii] > maxval) {                maxval = A[ii];            }            else {                result += maxval - A[ii];            }        }                return result;    }};