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. 

public class Solution {
    public int trap(int[] A) {
        int len = A.length;
        if(len<2) return 0;
        int []leftMax = new int[len], rightMax = new int[len];
        int max=0;
        max = A[0];
        leftMax[0] = 0;
        for(int i=1;i<len;i++){
            leftMax[i] = max;
            if(A[i]>max){
                max = A[i];
            }
        }
        max = A[len-1]; 
        rightMax[len-1]=0;
        int trap = 0;
        for(int i=len-2;i>=0;i--){
            rightMax[i] = max;
            if(max<A[i]){
                max = A[i];
            }
            if(Math.min(rightMax[i],leftMax[i])-A[i]>0){
                trap+=Math.min(rightMax[i],leftMax[i])-A[i];
            }
        }
        return trap;
    }
}