难度：简单。

正确解法：

class Solution {
public:
    double findMaxAverage(vector<int>& nums, int k) {
        int n = nums.size();
        if(n == 1)return nums[0];
        double average = 0, ans = 0;
        for(int i = 0; i < k; i++){
            average += nums[i];
        }
        average /= k;
        ans = average;
        int left = 1, right = k;
        while(right < n){
            average -= double(nums[left - 1]) / k;
            average += double(nums[right]) / k;
            if(ans < average){
                ans = average;
            }
            left++;
            right++;
        }
        return ans;
    }
};