LeetCode #24 Swap Nodes in Pairs

Question

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

For example,

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

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

Solution

Approach #1

/** * 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? {        let dummy = ListNode(0)        dummy.next = head        var p1: ListNode? = dummy        var p2 = p1?.next        while p2?.next != nil {            p1?.next = p2?.next            p2?.next = p2?.next?.next            p1?.next?.next = p2            p1 = p1?.next?.next            p2 = p1?.next        }        return dummy.next    }}

Time complexity: O(n).

Space complexity: O(1).

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