刷题：接雨水

Source : https://leetcode-cn.com/problems/trapping-rain-water/

算法思想： 每一个位置i所能接到的雨水取决于其两边的min（最大值）

class Solution {
    public int trap(int[] height) {
        if(height == null || height.length < 3) return 0;
        int n = height.length;
        int res = 0;
        int l=0,lm = 0,r=n-1, rm = 0;
        while(l<r) {
            lm = Math.max(lm, height[l]);
            rm = Math.max(rm, height[r]);
            if(lm < rm) 
                res += lm - height[l++];
            else
                res += rm - height[r--];
        }
        return res;
    }
}