力扣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* reverse(ListNode *head) {
         if (!head || !head -> next) {
            return head;
        }
        ListNode *pre = head, *cur = head -> next, *nxt = cur -> next;
        pre -> next = nullptr;
        cur -> next = pre;
        return nxt; //返回下一组的头节点
    }
    ListNode* swapPairs(ListNode* head) {
        if (!head || !head -> next) {
            return head;
        }
        ListNode *tail = head -> next;
        ListNode *nxt = reverse(head);
        head -> next = swapPairs(nxt);
        return tail;
    }
};