LeetCode 24. 两两交换链表中的节点(swift)

题目

给定一个链表，两两交换其中相邻的节点，并返回交换后的链表。你不能只是单纯的改变节点内部的值，而是需要实际的进行节点交换。 示例 1：

输入：head = [1,2,3,4]输出：[2,1,4,3]示例 2：输入：head = []输出：[]示例 3：输入：head = [1]输出：[1] 提示：链表中节点的数目在范围 [0, 100] 内0 <= Node.val <= 100

解题思路

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init() { self.val = 0; self.next = nil; }
 *     public init(_ val: Int) { self.val = val; self.next = nil; }
 *     public init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }
 * }
 */
class Solution {
    func swapPairs(_ head: ListNode?) -> ListNode? {
        guard let head = head, let next = head.next else {
        return head
    }

    let dummy = ListNode(-1, head)
    var prev: ListNode? = dummy
    var cur: ListNode? = prev?.next

    while cur != nil && cur?.next != nil {
        let nextNode = cur?.next
        cur?.next = nextNode?.next
        nextNode?.next = cur
        prev?.next = nextNode

        prev = cur
        cur = cur?.next
    }

    return dummy.next
    }
}