难度：easy
Suppose an array sorted in ascending order is rotated at some pivot unknown to you beforehand.
(i.e., 0 1 2 4 5 6 7 might become 4
5 6 7 0 1 2).
Find the minimum element.
You may assume no duplicate exists in the array.
思路：找到list中最小的一个数，开始想用两分法，后来觉得就遍历array，找到最小值岂不更省事。