let array = [5, 3, 1, -2, -4, -2, 2, 3, 5]

let result = array.enumerated().compactMap { (index, value) -> Int? in
if index > 0 {
let prevValue = array[index - 1]
if (prevValue > 0 && value < 0) || (prevValue < 0 && value > 0) {
return index - 1
}
}
return nil
}