24. Swap Nodes in Pairs

Description

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.

Solution

Iteration

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode prev = dummy;
        ListNode curr = head;
        
        while (curr != null && curr.next != null) {
            ListNode post = curr.next;
            prev.next = post;
            curr.next = post.next;
            post.next = curr;
            prev = curr;
            curr = curr.next;
        }
        
        return dummy.next;
    }
}

Recursion

其实使用两个指针就够了呀。

/**
 * 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 curr = head.next;
        head.next = swapPairs(curr.next);
        curr.next = head;
        return curr;
    }
}