Given an integer array arr that is guaranteed to be a mountain array, return the index of its peak element. The peak is the element that is strictly greater than its adjacent elements, where values strictly increase up to the peak and then strictly decrease after it.

Note: A mountain array has some index i such that arr[0] < arr[1] < ... < arr[i] and arr[i] > arr[i+1] > ... > arr[arr.length - 1].

Constraints:

• $3 \leq$ arr.length $\leq 10^5$

• $0 \leq$ arr[i] $\leq 10^6$

• arr is guaranteed to be a mountain array.

Input: arr = [0, 2, 5, 3, 1]-List[int]
Output: 2-int
Explanation: The array increases 0 < 2 < 5 and then decreases 5 > 3 > 1, so the peak element is 5 at index 2.

Input: arr = [1, 3, 8, 12, 4, 2]-List[int]
Output: 3-int
Explanation: The values strictly increase up to 12 and strictly decrease afterward, making 12 the peak at index 3.

Input: arr = [0, 10, 5, 2]-List[int]
Output: 1-int
Explanation: The array rises from 0 to 10 and then falls 10 > 5 > 2, so the peak index is 1.

Input: arr = [2, 4, 6, 9, 11, 7, 3, 1]-List[int]
Output: 4-int
Explanation: The sequence increases up to 11 (index 4) and then decreases, so index 4 is the peak.

Input: arr = [0, 1, 0]-List[int]
Output: 1-int
Explanation: This is the smallest valid mountain array (length 3), and the peak is 1 at index 1.

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