[LeetCode 024] 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.

Codes

Iterative version

public class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head == null) {
            return null;
        }
        ListNode anchor = new ListNode(0); // add a new node pointing to head
        anchor.next = head;
        ListNode pre = anchor; // the node before the pair
        ListNode next; // the node which the second node in pair points to
        ListNode node1; // the first node in pair
        while (pre.next != null && pre.next.next != null) {
            node1 = pre.next; // initial the first node
            next = pre.next.next.next; 
            node1.next.next = node1; // node1.next is the second node; set the second node pointing to the first node
            pre.next = node1.next; // set the node before the pair pointing to the second node 
            node1.next = next; // set the first node pointing to next node
            pre = pre.next.next; // initial pre node
        }
        return anchor.next;
    }
}

Recursive version

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

        ListNode newPair = head.next.next;
        ListNode newHead = head.next;
        head.next.next = head;
        head.next = swapPairs(newPair);
        return newHead;
    }
}