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;
     }
 }