Solution 1: Intuitive

func peakIndexInMountainArray(_ A: [Int]) -> Int {
    for i in stride(from: 1, to: A.count, by: 1) {
        if A[i] < A[i-1] {
            return i-1
        }
    }
    return -1
}

Solution 2: Binary Search

func peakIndexInMountainArray(_ A: [Int]) -> Int {
    var start = 0
    var end = A.count - 1
    while start < end {
        let mid = start + (end - start) / 2
        if A[mid] < A[mid+1] {
            start = mid + 1
        } else {
            end = mid
        }
    }
    return start
}