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).
0 1 2 4 5 6 7
4 5 6 7 0 1 2
Find the minimum element.
You may assume no duplicate exists in the array.