Create 643. Maximum Average Subarray I.md

643. Maximum Average Subarray I.md

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+# [643. Maximum Average Subarray I](https://leetcode.com/problems/maximum-average-subarray-i/description/)
+# 思路
+用一个sum记录当前窗口的和，向右移动窗口找出最大的sum  
+注意：亲测`if(max_sum < sum) max_sum = sum;`比`max_sum = max(max_sum, sum)`慢一些
+# C++
+```
+class Solution {
+public:
+    double findMaxAverage(vector<int>& nums, int k) {
+        double sum = 0, max_sum;
+        for(int i = 0; i < k; i++) sum += nums[i];
+        max_sum = sum;
+        for(int i = k; i < nums.size(); i++){
+            sum = sum - nums[i - k] + nums[i];
+            max_sum = max(sum, max_sum);
+        }
+        return max_sum / double(k);
+    }
+};
+```