_24.java

package com.fishercoder.solutions;

import com.fishercoder.common.classes.ListNode;

/** 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 _24 {

    public ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode second = head.next;
        ListNode third = second.next;
        second.next = head;
        head.next = swapPairs(third);
        return second;
    }

}