Swap Nodes in Pairs

Swap Nodes in Pairs.java

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) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if (head==null) return head;
        
        ListNode helper=new ListNode(0);
        
        helper.next=head;
        ListNode n1=helper;
        ListNode n2=head;
        
        while(n2!=null&& n2.next!=null){
            n1.next=n2.next;
            n2.next=n2.next.next;
            n1.next.next=n2;
            
            n1=n2;
            n2=n2.next;
            
            
        }
        return helper.next;
        
    }
}