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.

Solution

public class Solution {
    public int trap(int[] height) {
        int left = 0;
        int right = height.length - 1;
        int level = 0;

        int rain = 0;
        while(left < right) {
            if(height[left] < height[right]) {
                level = Integer.max(level, height[left]);
                rain += level - height[left];
                left++;
            } else {
                level = Integer.max(level, height[right]);
                rain += level - height[right];
                right--;
            }
        }

        return rain;
    }
}