题目链接：852. 山脉数组的峰顶索引

根据题目用复杂度用O(long n)的方法解决问题，我们可以想到用二分查找解决：

class Solution {
public:int peakIndexInMountainArray(vector<int>& arr) {int left=0,right=arr.size()-1;while(left <= right){//如果大于右边说明山峰在左边，小于右边边说明山峰在右边int mid = left+(right-left)/2;if(arr[mid] < arr[mid+1]){left = mid+1;}else if(arr[mid] > arr[mid+1]){right = mid - 1;}}return left;}
};