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!




public class Solution {
    public int trap(int[] A) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(A.length==0) return 0;
        int i = 0; int j = A.length-1;
        int lm = A[i]; int rm = A[j];
       
        int sum = 0;
       
        while(i<j){
            if(lm <= rm){
                if(A[++i] < lm) sum += lm - A[i];
                else lm = A[i];
            }
            else{
                if(A[--j] < rm) sum += rm - A[j];
                else rm = A[j];
            }
        }
        return sum;
    }
}