42.Trapping Rain Water (Hard) Google Amazon Bloomberg Twitter Apple

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], 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 Marcos for contributing this image!

    public int trap(int[] A) {
        // two pointers
        int start = 0, end = A.length - 1;
        int result = 0, leftmax = 0, rightmax = 0;

        while (start <= end) {
            leftmax = Math.max(leftmax, A[start]);
            rightmax = Math.max(rightmax, A[end]);
            // if leftmax < rightmax, so the (leftmax-A[start]) water can be stored
            if (leftmax < rightmax) {
                result += (leftmax - A[start]);
                start++;
            }
            else {
                result += (rightmax - A[end]);
                end--;
            }
        }
        return result;
    }