LeetCode 852. 山脉数组的峰顶索引（二分查找）

A.length >= 3
存在 0 < i < A.length - 1 使得A[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1]

示例 1：

输入：[0,1,0]
输出：1
示例 2：

输入：[0,2,1,0]
输出：1

class Solution {
public:
    int peakIndexInMountainArray(vector<int>& A) {
        int left = 1, right = A.size()-2, mid;
        while(left <= right)
        {
        	mid = left + ((right-left)>>1);
        	if(A[mid]>A[mid-1] && A[mid]>A[mid+1])
        		return mid;
        	else if(A[mid] < A[mid+1])
        		left = mid+1;
        	else
        		right = mid-1;
        }
        return -1;
    }
};