Trapping Rain Water

Question

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.

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!

Example:
Input: [0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6

LeetCodeleetcode.com/problems/trapping-rain-water

Solution

/*
@param: int[] height, height of bar;
@return: int, maximum rain water;
Algorithm: 保持两个指针从左右两端，逐渐逼近
*/
public int trap(int[] height) {
    if (height.length == 0) {
        return 0;
    }
    // two pointer, left and right;
    int left = 0, right = height.length - 1;
    int leftHeight = height[left];
    int rightHeight = height[right];
    
    int result = 0;
    while (left < right) {
        if (leftHeight < rightHeight) {
            left++;
            if (leftHeight < height[left]) {
                leftHeight = height[left];
            }else {
                result += leftHeight - height[left];
            }
        }else {
            right--;
            if (rightHeight < height[right]) {
                rightHeight = height[right];
            }else {
                result += rightHeight - height[right];
            }
        }
    }
    return result;
}