Swap Nodes in Pairs

recursion programming

/**
 * 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) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(head == null)
            return null;
        if(head.next == null)
            return head;
        ListNode first = head;
        ListNode second = head.next;
        ListNode third = second.next;
        second.next = first;
        first.next = swapPairs(third);
        return second;
    }
}