A peak element is an element that is greater than its neighbors.
Given an input array nums, where nums[i] ≠ nums[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 that nums[-1] = nums[n] = -∞.
Example 1:
Input: nums = [1,2,3,1]
Output: 2
Explanation: 3 is a peak element and your function should return the index number 2.
Example 2:
Input: nums = [1,2,1,3,5,6,4]
Output: 1 or 5
Explanation: Your function can return either index number 1 where the peak element is 2,
or index number 5 where the peak element is 6.
Note:
Your solution should be in logarithmic complexity.
思路：二分法
1、还是要逼近这个山峰值
2、如果nums【mid】》nums【mid+1】、则很有可能mid就是山峰值、所以end = mid
3、如果不是的话、则mid肯定不是山峰值、所以start = mid + 1
4、这样子start = end的时候肯定就是山峰值
5、因为左边是递增过去的、右边是山峰值
