LeetCode 24. 两两交换链表中的节点

迭代

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        ListNode* hair = new ListNode(-1,head);

        ListNode* pre = hair;

        while(pre->next != nullptr && pre->next->next != nullptr)
        {
            ListNode* node1 = pre->next;
            ListNode* node2 = pre->next->next;

            pre->next = node2;
            node1->next = node2->next;
            node2->next = node1;

            pre = pre->next->next;
        }

        return hair->next;
    }
};

递归

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if(head == nullptr || head->next == nullptr)
            return head;
        ListNode* newhead = head->next;
        head->next = swapPairs(newhead->next);
        newhead->next = head;
        return newhead;
    }
};