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.

Swap Nodes in Pairs.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
      if(head==null) return null; 
      if(head.next==null) return head; 
      ListNode previous=null; 
      ListNode current=head; 
      ListNode runner=head.next; 
      while(runner!=null && runner!=current)
      {
        current.next=runner.next; 
        runner.next=current; 
        if(previous==null)
        {
          previous=runner; 
          head=previous; 
          previous=previous.next; 
        }else
        {
          previous.next=runner; 
          previous=previous.next.next; 
        }
        current=current.next;
        if(current==null) break; 
        runner=runner.next.next.next; 
         
      }
      return head; 
    }
}

iterative method.