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.

/**
 * 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||head.next==null)
            return head;
        ListNode faked=new ListNode(0);
        faked.next=head;
        ListNode pre=faked;
        ListNode cur=head;
        while(cur!=null&&cur.next!=null){
            pre.next=cur.next;
            cur.next=pre.next.next;
            pre.next.next=cur;
            pre=cur;
            cur=pre.next;
        }
        return faked.next;
    }
}

LeetCode Solutions : Swap Nodes in Pairs