题解 | #交互链表节点#

class Solution {
public:
  // Recursive
  ListNode* swapPairs(ListNode* head) {
    // recursion exit condition
    if (!head || !head->next) return head;
    
    auto new_head = head->next;
    head->next = swapPairs(new_head->next);
    new_head->next = head;
    return new_head;
  }
  
  // Iterative
  ListNode* swapPairsII(ListNode* head) {
    ListNode dummy{0}, *pre = &dummy;
    dummy.next = head;

    while (pre->next && pre->next->next) {
      ListNode *p = pre->next, *q = p->next;
      pre->next = q;
      p->next = q->next;
      q->next = p;
      pre = p;
    }

    return dummy.next;
  }
  
};