LeetCode 042 Trapping Rain Water

题目要求：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!

代码如下：

class Solution {
public:
    int trap(int A[], int n) {

        int maxIdx = 0;
        int water = 0;

        //找到最长的木板，设为maxIdx
        for(int i = 1; i < n; i++){
            if(A[i] > A[maxIdx]){
                maxIdx = i;
            }
        }

        int max = A[0];

        //左侧逼近
        for(int i = 1; i < maxIdx; i++){

            if(max < A[i])
                max = A[i];
            //木板左边(max)和右边(最高)都比它高，则可以放
            // max - 该木板长度 的水
            else
                water += max - A[i];
        }

        //右侧逼近
        max = A[n - 1];
        for(int i = n - 2; i >= maxIdx; i--){
            if(max < A[i]) max = A[i];
            else water += max - A[i];
        }

        return water;

    }
};