swap-nodes-in-pairs.java

// Given a linked list, swap every two adjacent nodes and return its head.
//
//  
// Example 1:
//
//
// Input: head = [1,2,3,4]
// Output: [2,1,4,3]
//
//
// Example 2:
//
//
// Input: head = []
// Output: []
//
//
// Example 3:
//
//
// Input: head = [1]
// Output: [1]
//
//
//  
// Constraints:
//
//
// 	The number of nodes in the list is in the range [0, 100].
// 	0 <= Node.val <= 100
//
//
//  
// Follow up: Can you solve the problem without modifying the values in the list's nodes? (i.e., Only nodes themselves may be changed.)


/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if ((head == null)||(head.next == null)) return head;
        ListNode node = head.next;
        head.next = swapPairs(head.next.next); 
        node.next = head;
        return node;
    }
}