Swap Nodes in Pairs

Description

Given a linked list, swap every two adjacent nodes and return its head.

You may not modify the values in the list's nodes, only nodes itself may be changed.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Code

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head ==  null) return null;
        if (head.next == null) return head;
        
        ListNode newHead = swapPairs(head.next.next);
        
        ListNode temp = head.next;
        head.next.next = head;
        head.next = newHead;
        
        return temp;
    }
}