42. Trapping Rain Water（js）

42. 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.

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
题意：给定一个直方图，求出能积水的面积
代码如下：

/**
 * @param {number[]} height
 * @return {number}
 */
var trap = function(height) {
    var left=0,right=height.length-1;
    var maxLeft=0,maxRight=0;
    var res=0;
    while(left<right){
        if(height[left]<height[right]){
            if(height[left]>=maxLeft) maxLeft=height[left];
            else res+=maxLeft-height[left] ;
            left++;
        }else{
            if(height[right]>=maxRight) maxRight=height[right];
            else res+=maxRight-height[right];
            right--;
        }
    }
    return res;
};