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. 

1 public class Solution { 2     public int trap(int[] A) { 3         int len = A.length; 4         if(len<2) return 0; 5         int []leftMax = new int[len], rightMax = new int[len]; 6         int max=0; 7         max = A[0]; 8         leftMax[0] = 0; 9         for(int i=1;i
      
       max){12                 max = A[i];13             }14         }15         max = A[len-1]; 16         rightMax[len-1]=0;17         int trap = 0;18         for(int i=len-2;i>=0;i--){19             rightMax[i] = max;20             if(max
       
        0){24                 trap+=Math.min(rightMax[i],leftMax[i])-A[i];25             }26         }27         return trap;28     }29 }