643. Maximum Average Subarray I

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 = 4
Output: 12.75
Explanation: 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].

 這題的思維，要想像一個k大小的sub array 從 0開始一路前進到 vector的盡頭。

class Solution {
public:
double findMaxAverage(vector<int>& nums, int k)
{
    //先算出nums[0]到nums[k]的總和
    int sum = accumulate(nums.begin(), nums.begin() + k, 0), max_number = sum, n = nums.size();
    
    for (int i = k; i < n; i++)
    {
        //每前進一格就比一次大小
        sum += nums[i] - nums[i - k];
        max_number = max(max_number, sum);
    }
    
    return double(max_number) / k;
}
};