解析

//! 递归法，n,n
class Solution {
public:
  ListNode *swapPairs(ListNode *head) {
    if (head == nullptr || head->next == nullptr) {
      return head;
    }
    ListNode *node1 = head;
    ListNode *node2 = head->next;
    node1->next = swapPairs(node2->next);
    node2->next = node1;

    return node2;
  }
};

//! 迭代法，n,1
class Solution {
public:
  ListNode *swapPairs(ListNode *head) {
    ListNode *dummy = new ListNode(-1, head);
    ListNode *temp = dummy;
    while (temp->next != nullptr && temp->next->next != nullptr) {
      ListNode *node1 = temp->next;
      ListNode *node2 = temp->next->next;
      temp->next = node2;
      temp = node2->next;
      node2->next = node1;
      node1->next = temp;
      temp = node1;
    }
    ListNode *res = dummy->next;
    delete dummy;
    return res;
  }
};