leetcode Trapping Rain Water

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 fZ http://www.2cto.com/kf/ware/vc/" target="_blank" class="keylink">vciBjb250cmlidXRpbmcKIHRoaXMgaW1hZ2UhPC9wPgo8cD48L3A+CgoKCgoKPHA+PC9wPgo8cCBjbGFzcz0="action"> Discuss

Find the highest, then from the start to the highest, then the last to the highest..

class Solution {
 public:
  int trap(int A[], int n) {
    if (n <= 2)
      return 0;
    int i, maxElevation = A[0], maxIndex = 0, h = 0, res = 0;
    for (i = 1; i < n; ++i)
      if (A[i] > maxElevation) {
        maxElevation = A[i];
        maxIndex = i;
      }
    for (i = 0; i <= maxIndex - 1; ++i) 
      if (A[i] >= h)
        h = A[i];
      else
        res += (h - A[i]);
    
    h = 0;
    for (i = n - 1; i >= maxIndex + 1; --i) 
      if (A[i] >= h)
        h = A[i];
      else
        res += (h - A[i]);
    return res;
  }
};