【链表 - LeetCode】24. 两两交换链表中的节点

/**
 * 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 * dummyHead = new ListNode(0);
        dummyHead -> next = head;
        ListNode * tmp = dummyHead;
        ListNode*first, *second; // 反转前后用到的临时节点
        while(tmp->next!=nullptr && tmp->next->next!=nullptr){
            first = tmp->next;
            second = tmp->next->next;
            // 先调整tmp，再调整 first，再调整 second
            tmp -> next = second;
            first -> next = second -> next;
            second -> next = first;
            tmp = first;
        }
        ListNode * ans = dummyHead -> next;
        delete dummyHead;
        return ans;
    }
};

/**
 * 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;
    }
};