Given an array of n positive integers and a positive integer s, find the minimal length of a contiguous subarray of which the sum ? s. If there isn‘t one, return 0 instead.

For example, given the array [2,3,1,2,4,3] and s = 7,
the subarray [4,3] has the minimal length under the problem constraint.

思路：

类似于滑动窗口，维护一个长为k的窗口。

double findMaxAverage(vector<int>& nums, int k)
    {
        int s = accumulate(nums.begin(), nums.begin() + k, 0), m = s;
        for (int i = k; i < nums.size(); i++) {
            s += nums[i] - nums[i - k];
            m = max(m, s);
        }
        return double(m) / k;
    }

[leetcode-643-Maximum Average Subarray I]