852. Peak Index in a Mountain Array

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.

// Binary Search
int peakIndexInMountainArray(vector<int>& A) { // time: O(logn); space: O(1)
    int l = 0, r = A.size();
    while (l < r) {
        int mid = l + (r - l) / 2;
        if (A[mid] < A[mid + 1]) l = mid + 1;
        else r = mid;
    }
    return l;
}