leetcode 91: Trapping Rain Water

Trapping Rain WaterMar 10 '12

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
        
        int sz = A.length;
        if(sz<=2) return 0;
        
        int i=0, j=sz-1, lm=A[0], rm=A[j];
        int sum = 0;
        
        while(i<=j) {
            if(lm<rm) {
                if(A[i]<lm) {
                    sum += lm - A[i];
                } else {
                    lm = A[i];
                }
                ++i;
            } else {
                if(A[j]<rm) {
                    sum += rm - A[j];
                } else {
                    rm = A[j];
                }
                --j;
            }
        }
        
        return sum;
    }
}