Solution 1: Iterative

func reverseList(_ head: ListNode?) -> ListNode? {
    var pre: ListNode?
    var current = head
    while current != nil {
        let tmp = current?.next
        current?.next = pre
        pre = current
        current = tmp
    }
    return pre
}

Solution 2: Recursive

func reverseList(_ head: ListNode?) -> ListNode? {
    if head == nil || head?.next == nil {
        return head
    }
    let node = reverseList(head?.next)
    head?.next?.next = head
    head?.next = nil
    return node
}