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].

用一個sum變數當作一個大小為k的sliding window，紀錄區間內的和，先找到最大的區間和之後再除以k得到答案。

double findMaxAverage(vector<int>& nums, int k) { // time: O(n); space: O(1)
    double sum = 0;
    for (int i = 0; i < k; ++i) sum += nums[i];
    double res = sum;
    for (int i = k; i < nums.size(); ++i) {
        sum += nums[i] - nums[i - k];
        res = max(res, sum);
    }
    return res / k;
}

644. Maximum Average Subarray II