42. Trapping Rain Water

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!

Solution

public class Solution {
    public int trap(int[] height) {

        if (height.length == 0)
            return 0;

    int[] left = new int[height.length];
        int[] right = new int[height.length];

        int max = height[0];
        for(int i = 0; i < height.length; i ++) {
            left[i] = Math.max(height[i], max);
            max = left[i];
        }

        max = height[height.length - 1];
        for(int j = height.length - 1; j >= 0; j --) {
            right[j] = Math.max(height[j], max);
            max = right[j];
        }

        int result = 0;
        for(int i = 0; i < height.length; i ++) {
            int temp = Math.min(left[i], right[i]);
            if (temp > height[i])
                result += temp - height[i];
        }

        return result;

    }
}