class Solution {
    public int peakIndexInMountainArray(int[] A) {
        int i = 0, j = A.length-1;
        while(i <= j) {
            int m = i+(j-i)/2;
            if((m == 0 || A[m-1] < A[m]) && (m == A.length-1 || A[m+1] < A[m])) return m;
            if(m > 0 && A[m-1] > A[m]) j = m-1;
            else i = m+1;
        }
        return -1;
    }
}