643. Maximum Average Subarray I

题目：

Given an array consisting of n integers, find the contiguous subarray of given length k that has the maximum average value. And you need to output the maximum average value.

Example 1:

Input: [1,12,-5,-6,50,3], k = 4Output: 12.75Explanation: Maximum average is (12-5-6+50)/4 = 51/4 = 12.75

Note:

1. 1 <= k <= n <= 30,000.
2. Elements of the given array will be in the range [-10,000, 10,000].

思路：

本题一开始简单直接的思路是利用两层for循环，但是结果会time limit，那是每层循环都会有重复计算项，那么本题建立一个长度为k的窗口，从左至右依次滑动一个数字，每次的和就是加上最新的一个数，减去最左边的数。

代码：

class Solution {public:    double findMaxAverage(vector<int>& nums, int k) {        double res = -10000*k;        double sumnum = accumulate(nums.begin(),nums.begin()+k,0);        res = res<sumnum?sumnum:res;        for(int i=k;i<nums.size();i++)        {            sumnum = sumnum+nums[i]-nums[i-k];            res = res<sumnum?sumnum:res;        }        return res/k*1.0;    }};