class Solution { // 162. O(log N) O(1)
public:
    int findPeakElement(vector<int>& a) {
        int n = (int)a.size(), l = 0, r = n - 1;
        while (l <= r) {
            int m = l + ((r - l) >> 1);
            if ((m == 0 || a[m] > a[m-1]) && (m == n-1 || a[m] > a[m+1])) 
                return m;
            if (m < n-1 && a[m] < a[m+1]) 
                l = m + 1; 
            else 
                r = m - 1;
        }
        return l;
    }
};