#define _CRT_SECURE_NO_WARNINGS 1
#include<vector>
using namespace std;
class Solution {
public:
    int peakIndexInMountainArray(vector<int>& arr) {
        int left = 0, right = arr.size() - 1, mid = 0;    //置二分查找左右指针
        while (left < right)
        {
    mid = left + (right - left) / 2;
    if (arr[mid] < arr[mid + 1])left = mid + 1; //两段性为peak前arr[mid] < arr[mid + 1]
    else right = mid;        //peak后arr[mid] >arr[mid + 1]
           
        }
        return left;
    }
};