LeetCode #24 Swap Nodes in Pairs

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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