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[] height) {
        if (height == null || height.length == 0) {
        	return 0;
        }
        int res = 0;
        int[] left = new int[height.length];
        int[] right = new int[height.length];
        left[0] = height[0];
        int max = height[0];
        for (int i = 1; i < height.length; i++) {
        	 left[i] = Math.max(max, height[i]);
        	 max = Math.max(max, height[i]);
		}
        right[height.length-1] = height[height.length-1];
		max = height[height.length - 1];
		for (int i = height.length - 2; i >= 0; i--) {
			right[i] = Math.max(max, height[i]);
			max = Math.max(max, height[i]);
		}
		for (int i = 1; i < height.length-1; i++) {
			int area = Math.min(left[i], right[i]) - height[i];
			if (area > 0) {
				res += area;
			}
		}
       return res;
    }
}