leetcode 643[easy]--Maximum Average Subarray I

难度：easy

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].

思路：求给定list里面一段长度为K的片段，使得该片段的具有最大平均值。

           遍历全表，找到所有可能的长度为K的片段，求其最大平均值。

           关于ans的初始值设定，开始设为0，或者None 均为错误，于是设置为第一个K片段的平均值。