A peak element is an element that is greater than its neighbors.

Given an input array wherenum[i] ≠ num[i+1], find a peak element and return its index.

The array may contain multiple peaks, in that case return the index to any one of the peaks is fine.

You may imagine thatnum[-1] = num[n] = -∞.
For example, in array[1, 2, 3, 1], 3 is a peak element and your function should return the index number 2.      
logn的时间复杂度   二分查找