Swap Nodes in Pairs @LeetCode

package Level2;  
  
import Utility.ListNode;  
  
/** 
 * Swap Nodes in Pairs  
 *  
 *  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. 
 * 
 */  
public class S24 {  
  
    public static void main(String[] args) {  
        ListNode n1 = new ListNode(1);  
        ListNode n2 = new ListNode(2);  
        n1.next = n2;  
        ListNode n3 = new ListNode(3);  
        n2.next = n3;  
        n1.print();  
        ListNode head = swapPairs(n1);  
        head.print();  
    }  
      
    public static ListNode swapPairs(ListNode head) {  
        if(head == null){  
            return null;  
        }  
        // 当只有一个元素的情况  
        if(head.next == null){  
            return head;  
        }  
        ListNode i = head;  // i指向第1个  
        ListNode j = i.next;    // j指向第2个  
        ListNode k = j.next;    // k指向第3个  
          
        head = head.next;     
        while(j != null){  
            j.next = i;  
            if(k!=null && k.next!=null){    // 当有偶数个节点   
                i.next = k.next;  
            }else{      // 当有奇数个节点  
                i.next = k;  
            }  
              
            // 更新i，j，k的值，前进两格  
            i = k;  
            if(k != null){  
                j = k.next;  
            }else{  
                j = null;  
            }  
            if(k!=null && k.next!=null){  
                k = k.next.next;  
            }else{  
                k = null;  
            }  
        }  
        return head;  
    }  
}