A mountain array satisfies arr[0] < arr[1] < ... < arr[peak] > ... > arr[n-1]. Given such an array, return the index of the peak element using O(log n) time.

Example 1:

Input: 0 1 0
Output: 1

Example 2:

Input: 0 2 1 0
Output: 1

Input format: Space-separated integers (guaranteed mountain array).

Output format: Peak index.