Update 9. 用两个栈实现队列.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/9. 用两个栈实现队列.md

@@ -2,7 +2,7 @@
 
 ## 题目链接
 
-[牛客网](https://www.nowcoder.com/practice/54275ddae22f475981afa2244dd448c6?tpId=13&tqId=11158&tPage=1&rp=1&ru=/ta/coding-interviews&qru=/ta/coding-interviews/question-ranking&from=cyc_github)
+[Leetcode](https://leetcode.cn/problems/yong-liang-ge-zhan-shi-xian-dui-lie-lcof/)
 
 ## 题目描述
 
@@ -15,21 +15,28 @@ in 栈用来处理入栈（push）操作，out 栈用来处理出栈（pop）操
 <div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/3ea280b5-be7d-471b-ac76-ff020384357c.gif" width="450"/> </div><br>
 
 ```java
-Stack<Integer> in = new Stack<Integer>();
-Stack<Integer> out = new Stack<Integer>();
+class CQueue {
+    private Stack<Integer> in = new Stack<>();
+    private Stack<Integer> out = new Stack<>();
 
-public void push(int node) {
-    in.push(node);
-}
+    public CQueue() {
 
-public int pop() throws Exception {
-    if (out.isEmpty())
-        while (!in.isEmpty())
-            out.push(in.pop());
+    }
 
-    if (out.isEmpty())
-        throw new Exception("queue is empty");
+    public void appendTail(int value) {
+        in.push(value);
+    }
 
-    return out.pop();
+    public int deleteHead() {
+        if (out.isEmpty()) {
+            while (!in.isEmpty()) {
+                out.push(in.pop());
+            }
+        }
+        if (out.isEmpty()) {
+            return -1;
+        }
+        return out.pop();
+    }
 }
 ```