Givennnon-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], return6.

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 Marcosfor contributing this image!

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;
  }
};

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