Update 6. 从尾到头打印链表.md

题目链接改为力扣中的对应题目，题目描述和代码按该题的接口调整。

notes/6. 从尾到头打印链表.md

@@ -2,7 +2,7 @@
 
 ## 题目链接
 
-[牛客网](https://www.nowcoder.com/practice/d0267f7f55b3412ba93bd35cfa8e8035?tpId=13&tqId=11156&tPage=1&rp=1&ru=/ta/coding-interviews&qru=/ta/coding-interviews/question-ranking&from=cyc_github)
+[Leetcode](https://leetcode.cn/problems/cong-wei-dao-tou-da-yin-lian-biao-lcof/)
 
 ## 题目描述
 
@@ -17,14 +17,22 @@
 要逆序打印链表 1-\>2-\>3（3,2,1)，可以先逆序打印链表 2-\>3(3,2)，最后再打印第一个节点 1。而链表 2-\>3 可以看成一个新的链表，要逆序打印该链表可以继续使用求解函数，也就是在求解函数中调用自己，这就是递归函数。
 
 ```java
-public ArrayList<Integer> printListFromTailToHead(ListNode listNode) {
-    ArrayList<Integer> ret = new ArrayList<>();
-    if (listNode != null) {
-        ret.addAll(printListFromTailToHead(listNode.next));
-        ret.add(listNode.val);
-    }
+private ArrayList<Integer> tmp = new ArrayList<>();
+
+public int[] reverseBookList(ListNode head) {
+    recurse(head);
+    int[] ret = new int[tmp.size()];
+    for (int i = 0; i < ret.length; i++)
+        ret[i] = tmp.get(i);
     return ret;
 }
+
+private void recurse(ListNode head) {
+    if (head == null)
+        return;
+    recurse(head.next);
+    tmp.add(head.val);
+}
 ```
 
 ### 2. 使用头插法
@@ -33,7 +41,7 @@ public ArrayList<Integer> printListFromTailToHead(ListNode listNode) {
 
 链表的操作需要维护后继关系，例如在某个节点 node1 之后插入一个节点 node2，我们可以通过修改后继关系来实现：
 
-```java
+```text
 node3 = node1.next;
 node2.next = node3;
 node1.next = node2;
@@ -48,21 +56,23 @@ node1.next = node2;
 <div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/0dae7e93-cfd1-4bd3-97e8-325b032b716f-1572687622947.gif" width="420px"> </div><br>
 
 ```java
-public ArrayList<Integer> printListFromTailToHead(ListNode listNode) {
+public int[] reverseBookList(ListNode head) {
     // 头插法构建逆序链表
-    ListNode head = new ListNode(-1);
-    while (listNode != null) {
-        ListNode memo = listNode.next;
-        listNode.next = head.next;
-        head.next = listNode;
-        listNode = memo;
-    }
-    // 构建 ArrayList
-    ArrayList<Integer> ret = new ArrayList<>();
-    head = head.next;
+    ListNode dummy = new ListNode(-1);
+    int count = 0;
     while (head != null) {
-        ret.add(head.val);
-        head = head.next;
+        ListNode memo = head.next;
+        head.next = dummy.next;
+        dummy.next = head;
+        head = memo;
+        count++;
+    }
+    int[] ret = new int[count];
+    ListNode cur = dummy.next;
+    int idx = 0;
+    while (cur != null) {
+        ret[idx++] = cur.val;
+        cur = cur.next;
     }
     return ret;
 }
@@ -75,15 +85,16 @@ public ArrayList<Integer> printListFromTailToHead(ListNode listNode) {
 <div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/9d1deeba-4ae1-41dc-98f4-47d85b9831bc.gif" width="340px"> </div><br>
 
 ```java
-public ArrayList<Integer> printListFromTailToHead(ListNode listNode) {
+public int[] reverseBookList(ListNode head) {
     Stack<Integer> stack = new Stack<>();
-    while (listNode != null) {
-        stack.add(listNode.val);
-        listNode = listNode.next;
+    while (head != null) {
+        stack.push(head.val);
+        head = head.next;
     }
-    ArrayList<Integer> ret = new ArrayList<>();
+    int[] ret = new int[stack.size()];
+    int idx = 0;
     while (!stack.isEmpty())
-        ret.add(stack.pop());
+        ret[idx++] = stack.pop();
     return ret;
 }
 ```