42. 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], return6.

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!

思路：双指针从两边往中间找

    public int trap(int[] height) {
        if (height.length<3){
            return 0;
        }

        int result = 0;
        int left = 0;
        int right = height.length-1;

        while(left<right&&height[left]<=height[left+1]) left++;
        while(left<right&&height[right]<=height[right-1]) right--;

        while(left<right){
            int height_left = height[left];
            int height_right = height[right];
            if (height_left<=height_right){
                while (left<right && height_left>=height[++left]){
                    result += height_left - height[left];
                }
            }else{
                while (left<right && height[--right]<=height_right){
                    result += height_right - height[right];
                }
            }
        }

        return result;
    }
}