Swift 删除链表的倒数第N个节点 - LeetCode

给定一个链表: 1->2->3->4->5, 和 n = 2.

当删除了倒数第二个节点后，链表变为 1->2->3->5.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.next = nil
 *     }
 * }
 */
class Solution {
  func removeNthFromEnd(_ head: ListNode?, _ n: Int) -> ListNode? {
    var first = head
    for _ in 0..<n {
        first = first?.next
    }
    if first == nil {
        return head?.next
    }
    var sec = head
    while first?.next != nil {
        first = first?.next
        sec = sec?.next
    }
    sec?.next = sec?.next?.next
    return head
  }
}