LinkedList-24-Swap Nodes in Pairs

Description:

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:

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode swapPairs(ListNode head) {
if (head == null || head.next == null) return head;
ListNode cur = head;
ListNode newHead = head.next;
while (cur != null && cur.next != null) {
ListNode tmp = cur;
cur = cur.next;
tmp.next = cur.next;
cur.next = tmp;
cur = tmp.next;
if (cur != null && cur.next != null) tmp.next = cur.next;
}
return newHead;
}
}

Recursive Version:

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