二分查找（C++实现）

class Solution {

public:

    int peakIndexInMountainArray(vector<int>& arr) {

        int n = arr.size();

        if(n==0)

        {

            return -1;

        }

        int left = 0;

        int right = n-1;

        while(left<right)

        {

            int mid = left + (right-left)/2;

            if(arr[mid]<arr[mid+1])

            {

                left = mid + 1;

            }

            else{

                right = mid;

            }

        }

        return left;

    }

};