42. Trapping Rain Water(直方图 存水量 hard)

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.

class Solution {
    public int trap(int[] height) {
        int left = 0;
        int right = height.length-1;
        int trap = 0;
        int h = 0;
        while(left<right){
            if(height[left]<height[right]){
                h = Math.max(height[left],h);
                trap+=(h - height[left]);
                left++;
            }
            else{
                h = Math.max(height[right],h);
                trap+=(h - height[right]);
                right--;
                
            }
        }
        return trap;
    }
}