Find a peak element

• Time : $O(logn)$
• Space : $O(1)$

int Solution::solve(vector<int> &A) {
    if(A.size() == 0) return A[0];
    int l = 0, r = A.size() - 1;
    while(l <= r) {
        int m = l + (r - l) / 2;
        if(m == 0) return A[0] > A[1] ? A[0] : A[1];
        if(m == A.size() - 1) return A[A.size() - 1] > A[A.size() - 2] ? A[A.size() - 1] : A[A.size() - 2];
        if(A[m-1] < A[m] and A[m] > A[m+1]) return A[m];
        if(A[m] < A[m + 1]) l = m + 1;
        else r = m - 1;
    }
    return -1;
}