Trapping rain water

public class Solution {
    /**
     * @param heights: an array of integers
     * @return: a integer
     */
    public int trapRainWater(int[] heights) {
        // write your code here
        if (heights == null || heights.length <= 2) {
            return 0;
        }

        int res = 0;
        int left = 0;
        int right = heights.length-1;

        int lmax = heights[left];
        int rmax = heights[right];

        while (left < right) {
            if (lmax < rmax) {
                if (left + 1 < heights.length && heights[left+1] < lmax) {
                    res += lmax - heights[left+1];
                }
                left++;
                lmax = Math.max(lmax, heights[left]);
            } else {
                if (right - 1 >= 0 && heights[right-1] < rmax) {
                    res += rmax - heights[right-1];
                }
                right--;
                rmax = Math.max(rmax, heights[right]);
            }
        }

        return res;
    }
}