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# 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());
    }
  }

}
```


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