024.Swap Nodes in Pairs

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.

Iterative Solution, O(n) time O(1) space

public class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode prev = dummy;
        ListNode first, second;
        
        while (prev.next != null && prev.next.next != null){
            first = prev.next;
            second = prev.next.next;
            first.next = second.next;
            prev.next = second;
            second.next = first;
            prev = prev.next.next;
        }
        
        return dummy.next;
    }
}

Recursive Solution, O(n) time O(n) space

public class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode second = head.next;
        head.next = swapPairs(head.next.next);
        second.next = head;
        return second;
    }
}