Java基礎(chǔ)教程欄目介紹如何用兩個棧實現(xiàn)一個隊列。
隊列和棧是計算機中兩個非常重要的數(shù)據(jù)結(jié)構(gòu)，經(jīng)過前面的學習（《隊列》、《棧》）我們知道了它們各自的特點，隊列是先進先出（FIFO）的，而棧是先進后出（FILO）的，那如何用棧來實現(xiàn)隊列呢？這可是一道經(jīng)典的面試題，所以本文我們就來實現(xiàn)一下。
在正式開始之前，我們先來回顧一下棧和隊列的常用方法。
棧（Stack）的常用方法包含以下這些：
- push()：入棧方法，向棧頂添加元素；
- pop()：出棧方法，將棧頂?shù)脑匾瞥⒎祷卦兀?/li>
- peek()：查詢棧頂元素，并不會移除元素。
隊列（Queue）的常用方法包含以下這些：
- offer()：入隊方法，向隊尾添加元素；
- poll()：出隊方法，從隊頭移除并返回元素；
- peek()：查詢隊頭元素，并不會移除元素。
有了這些前置知識，接下來我們來看今天的題目。
題目描述
用兩個棧實現(xiàn)一個隊列。隊列的聲明如下，請實現(xiàn)它的兩個函數(shù) appendTail 和 deleteHead，分別完成在隊列尾部插入整數(shù)和在隊列頭部刪除整數(shù)的功能，若隊列中沒有元素，deleteHead?操作返回 -1。
示例 1：
輸入：
["CQueue","appendTail","deleteHead","deleteHead"]
[[],[3],[],[]]
輸出：[null,null,3,-1]
示例 2：
輸入：
["CQueue","deleteHead","appendTail","appendTail","deleteHead","deleteHead"]
[[],[],[5],[2],[ ],[]]
Output: [null,-1,null,null,5,2]
Prompt:
1
Up to 10000 calls to appendTail and deleteHead will be made
leetcode: leetcode-cn.com/problems/yo…
Solution Question idea
The meaning of this question is actually easy to understand, which is to change the first-in-last-out stack to a first-in-first-out queue. In fact, the question also gives some tips, "Use two stacks to Implement a queue". The core idea of ????this question is "a negative makes a positive". We first use a stack to store elements (the first element to enter is at the bottom of the stack), and then add the elements in the first stack Move to the new stack. At this time, the first entered element is on the top of the stack. Then when the second stack is used to pop it out, the entire execution order becomes first-in, first-out.
Next, let’s implement the entire process graphically.
Step 1
First push the element into the first stack, as shown in the following figure: 
Step 2
Push the element into the first stack The elements in one stack are moved to the second stack, as shown in the following figure: 
Step 3
All elements are popped from the second stack, as shown in the following figure Shown: 
Summary
As can be seen from the above picture, the order of adding elements is 1, 2, 3, and the final order of popping out the stack after passing through two stacks is also 1, 2 , 3, so that we implement the queue (first in, first out) through two stacks. 
Implementation code
Next we will use code to implement the above ideas:
class?CQueue?{
????Stack<integer>?inputStack;?//?入棧的容器（添加時操作)
????Stack<integer>?outputStack;?//?出棧和查詢的棧容器
????public?CQueue()?{
????????inputStack?=?new?Stack();
????????outputStack?=?new?Stack();
????}????//?添加操作
????public?void?appendTail(int?value)?{
????????inputStack.push(value);
????}????//?刪除操作
????public?int?deleteHead()?{????????if?(!outputStack.isEmpty())?{????????????//?出棧容器不為空
????????????return?outputStack.pop();
????????}?else?if?(!inputStack.isEmpty())?{????????????//?入棧?stack?全部轉(zhuǎn)移到出棧?stack
????????????while?(!inputStack.isEmpty())?{
????????????????outputStack.push(inputStack.pop());
????????????}
????????}????????return?outputStack.isEmpty()???-1?:?outputStack.pop();
????}
}復制代碼</integer></integer>
We submit the above test code in LeetCode, and the execution results are as follows:
Notes
There are two small details that require special attention during the entire implementation process:
- The first stack is only responsible for pushing into the stack (temporarily Store data), the second stack is only responsible for popping (the final queue execution sequence);
- Every time stack 2 is popped, all elements must be popped out before appending from stack 1 (Add) new data. When all the data in stack 2 are not pushed out of the stack, the elements from stack 1 cannot be pushed into stack 2. This will cause the execution order of the elements to be confused.
Summary
In this article, we use two first-in-last-out stacks and implement the first-in-first-out feature of the queue through the idea of ????"negatives make positives", but special attention is required. When the second stack, which is the pop-up container, is not empty (stack), the elements in the first stack cannot be added to the second stack to avoid confusing the program execution order.
Related free learning recommendations: java basic tutorial
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