【LeetCode】24. 两两交换链表中的节点

链接：

https://leetcode-cn.com/problems/swap-nodes-in-pairs

描述：

给定一个链表，两两交换其中相邻的节点，并返回交换后的链表。
你不能只是单纯的改变节点内部的值，而是需要实际的进行节点交换。

示例:
给定 1->2->3->4, 你应该返回 2->1->4->3.

ListNode* swapPairs(ListNode* head) {}

思路：递归

C++

展开后查看

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

Java

展开后查看

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

思路：非递归

C++

展开后查看

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        ListNode* dummyHead = new ListNode(-1);
        dummyHead->next = head;
        ListNode* p = dummyHead;
        while(p->next != NULL && p->next->next != NULL){
            ListNode* node1 = p->next;
            ListNode* node2 = p->next->next;
            node1->next = node2->next;
            node2->next = node1;
            p->next = node2;
            p = node1;
        }
        p = dummyHead->next;
        delete dummyHead;
        return p;
    }
};

Java

展开后查看

/**
 * 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 dummyHead = new ListNode(-1);
        dummyHead.next = head;
        ListNode p = dummyHead;
        while(p.next != null && p.next.next != null){
            ListNode node1 = p.next;
            ListNode node2 = p.next.next;
            node1.next = node2.next;
            node2.next = node1;
            p.next = node2;
            p = node1;
        }
        return dummyHead.next;
    }
}