Swap Nodes in Pairs
Given a linked list, swap every two adjacent nodes and return its head.
Example
Given 1->2->3->4, you should return the list as 2->1->4->3.
Challenge
Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 | /** * Definition for singly-linked list. * public class ListNode { * int val; * ListNode next; * ListNode(int x) { val = x; } * } */ public class Solution { /** * @param head a ListNode * @return a ListNode */ public ListNode swapPairs(ListNode head) { // Write your code here ListNode dummy = new ListNode(0); dummy.next = head; head = dummy; if (head == null) { return null; } while(head.next != null && head.next.next != null) { ListNode n1 = head.next; ListNode n2 = head.next.next; head.next = n2; n1.next = n2.next; n2.next = n1; head = n1; } return dummy.next; } } |