Swift-链表的倒数第k个节点

题目:输入一个链表，输出该链表中倒数第k个结点。为了符合大多数人的习惯，本题从1开始计数，即链表的尾结点是倒数第1个结点。例如一个链表有6个结点，从头结点开始它们的值依次是1、2、3、4、5、6。这个链表的倒数第3个结点是值为4的结点。
核心代码:

    func findNodeByDescendIndex(head:ListNode?,index:Int) -> ListNode? {
        if head == nil || index < 1 {
            return nil
        }
        var preNode:ListNode? = head
        var firstNode:ListNode? = head
        
        for i in 0..<index-1 {
            if preNode == nil {
                break
            }
            preNode = preNode?.next
        }
        
        while preNode?.next != nil {
            preNode = preNode?.next
            firstNode = firstNode?.next
        }
        return firstNode
    }

测试代码:

var searchManager:ListNodeManager = ListNodeManager()
var headNode:ListNode?
var searchHeadNode:ListNode?
searchManager.createList(&headNode, data: 1)
searchHeadNode = headNode

searchManager.createList(&headNode, data: 2)
searchManager.createList(&headNode, data: 3)
searchManager.createList(&headNode, data: 4)
searchManager.createList(&headNode, data: 5)
searchManager.createList(&headNode, data: 6)
searchManager.createList(&headNode, data: 7)
searchManager.createList(&headNode, data: 8)
searchManager.createList(&headNode, data: 9)

var searchNode:SearchNode = SearchNode()
var resultNode:ListNode? = searchNode.findNodeByDescendIndex(head: searchHeadNode, index: 3)
if resultNode != nil {
    print("FlyElephant-节点的Value--\(resultNode!.value!)")
} else {
    print("节点没有值")
}