From 91b8ac05f7354b3effface2eebc263346d13d330 Mon Sep 17 00:00:00 2001
From: Sainnhe Park <i@sainnhe.dev>
Date: Fri, 4 Nov 2022 16:56:13 +0800
Subject: [PATCH] s0019

---
 ...s0019_remove_nth_node_from_end_of_list.hpp | 17 +++++++++++
 ...s0019_remove_nth_node_from_end_of_list.cpp | 28 +++++++++++++++++++
 2 files changed, 45 insertions(+)
 create mode 100644 include/s0019_remove_nth_node_from_end_of_list.hpp
 create mode 100644 src/s0019_remove_nth_node_from_end_of_list.cpp

diff --git a/include/s0019_remove_nth_node_from_end_of_list.hpp b/include/s0019_remove_nth_node_from_end_of_list.hpp
new file mode 100644
index 0000000..95109f4
--- /dev/null
+++ b/include/s0019_remove_nth_node_from_end_of_list.hpp
@@ -0,0 +1,17 @@
+#ifndef S0019_REMOVE_NTH_NODE_FROM_END_OF_LIST
+#define S0019_REMOVE_NTH_NODE_FROM_END_OF_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* removeNthFromEnd(ListNode* head, int n);
+};
+
+#endif
diff --git a/src/s0019_remove_nth_node_from_end_of_list.cpp b/src/s0019_remove_nth_node_from_end_of_list.cpp
new file mode 100644
index 0000000..f462fcd
--- /dev/null
+++ b/src/s0019_remove_nth_node_from_end_of_list.cpp
@@ -0,0 +1,28 @@
+#include "s0019_remove_nth_node_from_end_of_list.hpp"
+
+// 思路一：遍历两次，第一次遍历得到总长度，第二次遍历找出倒数第 n 个
+// 思路二：栈，先进后出，全部入栈后弹出倒数第 n 个把它舍弃掉
+// 思路三：双指针，快指针比慢指针多走 n
+// 步，等快指针走到头时，慢指针指向的的就是倒数第 n 个
+
+// 双指针实现
+ListNode* Solution::removeNthFromEnd(ListNode* head, int n) {
+  // 一种常见的处理头指针的方法是添加一个哑指针，这个哑指针指向头指针
+  // 这样我们就不需要单独讨论头指针了。
+  ListNode dummy(0, head);
+  ListNode* fast = &dummy;
+  for (int i{0}; i < n; i++) {
+    if (fast == nullptr) {
+      return nullptr;
+    } else {
+      fast = fast->next;
+    }
+  }
+  ListNode* slow = &dummy;
+  while (fast->next != nullptr) {
+    fast = fast->next;
+    slow = slow->next;
+  }
+  slow->next = slow->next->next;
+  return dummy.next;
+}