leetcode+ 固定区间长和平均最大，固定窗口，然后滑动。

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class Solution {
public:
    double findMaxAverage(vector<int>& nums, int k) {
        int maxSum =0 , sum =0;
        for(int i=0; i<k;i++){
            sum+=nums[i];
        }
        maxSum = sum;
        for(int l=0, r=k; r<nums.size(); l++, r++){
            sum += nums[r] - nums[l];
            maxSum = max(maxSum, sum);
        }
        return double(maxSum)/k;
    }
};