[Swift]LeetCode24. 两两交换链表中的节点 | Swap Nodes in Pairs

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Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Note:

• Your algorithm should use only constant extra space.
• You may not modify the values in the list's nodes, only nodes itself may be changed.

---

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。

示例:

给定 1->2->3->4, 你应该返回 2->1->4->3.

说明:

• 你的算法只能使用常数的额外空间。
• 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。

---

8ms

/**
 * 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 swapPairs(_ head: ListNode?) -> ListNode? {
        var current = head
        
        let result = head?.next ?? head
        
        while let next = current?.next {
            let nextNext = next.next
            next.next = current
            current?.next = nextNext?.next ?? nextNext
            current = nextNext
        }
        
        return result
    }
}

---

12ms

class Solution {
    func swapPairs(_ head: ListNode?) -> ListNode? {
        var current = head
        var next = current?.next
        while next != nil {
            (current!.val, next!.val) = (next!.val, current!.val)

            current = current?.next?.next
            next = current?.next
        }
        return head
    }
}

---

20ms

/**
 * 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 swapNodes(pre: ListNode?, first: ListNode) {
    let temp = first.next?.next
    if pre != nil {
        pre?.next = first.next

    }
    if first.next?.next != nil {
        first.next?.next = first
        first.next = temp
    } else {
        first.next?.next = first
        first.next = nil
    }
}

func swapPairs(_ head: ListNode?) -> ListNode? {
    var pre: ListNode? = nil
    var result = head
    if let root = head {
        if root.next != nil {
            result = root.next
        }
        var current: ListNode? = head
        while(current?.next != nil) {
            swapNodes(pre: pre, first: current!)
            pre = current
            current = current?.next
        }
    }

    return result
}
}

---

24ms

/**
 * 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 swapPairs(_ head: ListNode?) -> ListNode? {
        guard let head = head else {
            return nil
        }

        var left: ListNode? = head
        var right = head.next
        let root = head.next
        while left != nil {
            let temp = right?.next
            right?.next = left
            left?.next = temp?.next ?? temp

            left = temp
            right = temp?.next
        }

        return root ?? head
    }
}