Peak Index in a Mountain Array

You are given an integer **mountain** array `arr` of length `n` where the values increase to a **peak element** and then decrease. Return the index of the peak element. Your task is to solve it in `O(log(n))` time complexity.   **Example 1:** **Input:** arr = [0,1,0] **Output:** 1 **Example 2:** **Input:** arr = [0,2,1,0] **Output:** 1 **Example 3:** **Input:** arr = [0,10,5,2] **Output:** 1   **Constraints:** - `3 <= arr.length <= 105` - `0 <= arr[i] <= 106` - `arr` is **guaranteed** to be a mountain array.