Let's call an array A a mountain if the following properties hold:

• A.length >= 3
• There exists some 0 < i < A.length - 1 such that A[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1]

Given an array that is definitely a mountain, return any i such that A[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1]

Example 1:

Input: [0,1,0]
Output: 1

Example 2:

Input: [0,2,1,0]
Output: 1

Note:

1. 3 <= A.length <= 10000
2. 0 <= A[i] <= 10^6
3. A is a mountain, as defined above.

分析：題目給的陣列有遞增和遞減兩部分，根據這個規律，用二分查詢法即可。

程式碼：

//leetcode刷分技巧
static const auto _ = []() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    return nullptr;
}();

class Solution {
public:
    int peakIndexInMountainArray(vector<int>& A) {
        int left = 0, right = A.size()-1;
        int mid = (left+right)/2;
        while(true)
        {
            if((mid==0)||(mid==A.size()-1))
                return mid;
            if(A[mid]>=A[mid-1])
                if(A[mid]>=A[mid+1])
                    return mid;
                else
                {
                    left = mid+1;
                    mid = (left+right)/2;
                }
            else
            {
                right = mid-1;
                mid = (left + right)/2;
            }
        }
    }
};