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.

 

Iterative Vestion:

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) { val = x; }
 7  * }
 8  */
 9 public class Solution {
10     public ListNode swapPairs(ListNode head) {
11         if (head == null || head.next == null) {
12             return head;
13         }
14         ListNode dummy = new ListNode(0);
15         dummy.next = head;
16         head = dummy;
17         while (head.next != null && head.next.next != null) {
18             ListNode n1 = head.next;
19             ListNode n2 = head.next.next;
20             n1.next = n2.next;
21             head.next = n2;
22             n2.next = n1;
23             head = n1;
24         }
25         return dummy.next;
26         
27     }
28 }

 

Recursive Version

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) { val = x; }
 7  * }
 8  */
 9 public class Solution {
10     public ListNode swapPairs(ListNode head) {
11         if (head == null || head.next == null) {
12             return head;
13         }
14         ListNode tail = swapPairs(head.next.next);
15         ListNode temp = head.next;
16         head.next = tail;
17         temp.next = head;
18         return temp;
19         
20     }
21 }

 

posted @ 2016-08-14 09:45  YuriFLAG  阅读(129)  评论(0)    收藏  举报