# 232.Implement-Queue-using-Stacks

## 232. Implement Queue using Stacks

## 题目地址

<https://leetcode.com/problems/implement-queue-using-stacks/>

<https://www.lintcode.com/problem/implement-queue-by-two-stack>

## 题目描述

```
Implement the following operations of a queue using stacks.

push(x) -- Push element x to the back of queue.
pop() -- Removes the element from in front of queue.
peek() -- Get the front element.
empty() -- Return whether the queue is empty.

Example:
MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

Notes:
You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).
```

## 代码

```java
public class Solution {
  private Stack<Integer> stack1;
  private Stack<Integer> stack2;

  public Solution() {
    stack1 = new Stack<Integer>();
    stack2 = new Stack<Integer>();
  }

  public void push(int element) {
    stack1.push(element);
  }

  public int pop() {
    if (stack2.empty()) {
      stack1ToStack2(stack1, stack2);
    }
    return stack2.pop();
  }

  public int top() {
    if (stack2.empty()) {
      stack1ToStack2(stack1, stack2);
    }
    return stack2.peek();
  }

  private void stack1ToStack2() {
    while (!stack1.empty()) {
      stack2.push(stack1.pop());
    }
  }

}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://wentao-shao.gitbook.io/leetcode/data-structure/232.implement-queue-using-stacks.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.