From 4ae4313646007d73a36d4728aec4c11b559f7439 Mon Sep 17 00:00:00 2001
From: Sainnhe Park <i@sainnhe.dev>
Date: Fri, 2 Dec 2022 16:15:15 +0800
Subject: [PATCH] s0024

---
 src/s0024_swap_nodes_in_pairs.cpp | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/s0024_swap_nodes_in_pairs.cpp b/src/s0024_swap_nodes_in_pairs.cpp
index e252ca8..34fdbe1 100644
--- a/src/s0024_swap_nodes_in_pairs.cpp
+++ b/src/s0024_swap_nodes_in_pairs.cpp
@@ -1,11 +1,17 @@
 #include "s0024_swap_nodes_in_pairs.hpp"
 
 ListNode* S0024::swapPairs(ListNode* head) {
+  // 只有当 head 和 head->next 都不为空时才翻转
+  // 其它情况都直接返回
   if (head == nullptr || head->next == nullptr) {
     return head;
   }
-  ListNode *newHead = head->next;
-  head->next = swapPairs(newHead->next);
-  newHead->next = head;
-  return newHead;
+  // 从这一行开始就假设 head 和 head->next 都不为空了
+  // 我们首先把从 head->next->next 开始的链表翻转了
+  ListNode* ptr = swapPairs(head->next->next);
+  // 现在我们开始翻转 head 和 head->next
+  ListNode* ans = head->next;
+  head->next->next = head;
+  head->next = ptr;
+  return ans;
 }