From 7bc754ae039218b77ff69deb8f29256b50238f62 Mon Sep 17 00:00:00 2001
From: Sainnhe Park <i@sainnhe.dev>
Date: Fri, 2 Dec 2022 16:06:34 +0800
Subject: [PATCH] s0206

---
 include/s0206_reverse_linked_list.hpp | 18 +++++++++++++
 src/s0206_reverse_linked_list.cpp     | 37 +++++++++++++++++++++++++++
 2 files changed, 55 insertions(+)
 create mode 100644 include/s0206_reverse_linked_list.hpp
 create mode 100644 src/s0206_reverse_linked_list.cpp

diff --git a/include/s0206_reverse_linked_list.hpp b/include/s0206_reverse_linked_list.hpp
new file mode 100644
index 0000000..664ed85
--- /dev/null
+++ b/include/s0206_reverse_linked_list.hpp
@@ -0,0 +1,18 @@
+#ifndef S0206_REVERSE_LINKED_LIST_HPP
+#define S0206_REVERSE_LINKED_LIST_HPP
+
+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 S0206 {
+ public:
+  ListNode* reverseList1(ListNode* head);
+  ListNode* reverseList2(ListNode* head);
+};
+
+#endif
diff --git a/src/s0206_reverse_linked_list.cpp b/src/s0206_reverse_linked_list.cpp
new file mode 100644
index 0000000..c623529
--- /dev/null
+++ b/src/s0206_reverse_linked_list.cpp
@@ -0,0 +1,37 @@
+#include "s0206_reverse_linked_list.hpp"
+
+// 双指针法
+ListNode* S0206::reverseList1(ListNode* head) {
+  if (head == nullptr) {
+    return nullptr;
+  }
+  ListNode* pre = head;
+  ListNode* cur = head->next;
+  pre->next = nullptr;
+  ListNode* curNext;
+  while (cur != nullptr) {
+    curNext = cur->next;
+    cur->next = pre;
+    pre = cur;
+    cur = curNext;
+  }
+  return pre;
+}
+
+// 递归法
+ListNode* S0206::reverseList2(ListNode* head) {
+  if (head == nullptr) {
+    return nullptr;
+  } else if (head->next == nullptr) {
+    return head;
+  }
+  // 翻转从第二个节点开始的链表
+  // 翻转完后 head->next 指向这个链表的末尾，而这个链表的末尾元素的 next
+  // 指向空指针
+  ListNode* last = reverseList2(head->next);
+  // 把这个链表末尾元素的 next 指向 head
+  head->next->next = head;
+  // 把 head->next 指向空指针
+  head->next = nullptr;
+  return last;
+}