19.删除链表的倒数第N个节点

https://leetcode-cn.com/problems/remove-nth-node-from-end-of-list/

func removeNthFromEnd(_ head: ListNode?, _ n: Int) -> ListNode? {
    let dummyNode = ListNode(0)
    dummyNode.next = head
    var firstNode = dummyNode
    var secondNode = dummyNode
    var count = n
    while n != 0 {
        firstNode = firstNode.next!
        count = count - 1
    }
    while firstNode.next != nil {
        firstNode = firstNode.next!
        secondNode = secondNode.next!
    }
    secondNode.next = secondNode.next?.next
    return dummyNode.next
}