[LeetCode][JavaScript]Trapping Rain Water

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!

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

---

接雨水，双指针。

一开始双指针分别指向头和尾。

每轮循环找出两边最高的bar，这一轮可以确定的容积取决于短的那条，因为无论中间隔着什么，两边的max总能保证可以存下这些水。

/**
 * @param {number[]} height
 * @return {number}
 */
var trap = function(height) {
    var i = 0, j = height.length - 1, res = 0,
        leftMax = height[i], rightMax = height[j];
    while(i <= j){
        leftMax = Math.max(leftMax, height[i]);
        rightMax = Math.max(rightMax, height[j]);
        if(leftMax > rightMax){
            res += rightMax - height[j];
            j--;
        }else{
            res += leftMax - height[i];
            i++;
        }

    }
    return res;
};