LeetCode 42. Trapping Rain Water（装雨水）

原题网址：https://leetcode.com/problems/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!

方法：从左右两边夹逼，保持两边各自的最大值。

public class Solution {
    public int trap(int[] height) {
        if (height == null || height.length <= 2) return 0;
        int i=0, j=height.length-1;
        int left=height[i], right=height[j];
        int sum = 0;
        while (i+1 right) {
                j--;
                sum += Math.max(0, right-height[j]);
                right = Math.max(right, height[j]);
            }
        }
        return sum;
    }
}