A peak element is an element that is strictly greater than its neighbors.

Given an integer array `nums`, find a peak element, and return its index. If the array contains multiple peaks, return the index to any of the peaks.

You may imagine that `nums[-1] = nums[n] = -∞`.

You must write an algorithm that runs in `O(log n)` time.

Example 1:

    Input: nums = [1,2,3,1]
    Output: 2
    Explanation: 3 is a peak element and your function should return the index number 2.

Example 2:

    Input: nums = [1,2,1,3,5,6,4]
    Output: 5
    Explanation: Your function can return either index number 1 where the peak element is 2, or index number 5 where the peak element is 6.

Constraints:

- `1 <= nums.length <= 1000`
- `-231 <= nums[i] <= 231 - 1`
- `nums[i] != nums[i + 1]` for all valid `i`.