> For the complete documentation index, see [llms.txt](https://nataliekung.gitbook.io/ladder_code/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://nataliekung.gitbook.io/ladder_code/chapter1/implement-queue-using-stacks.md).

# Implement Queue using Stacks

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.

分析：

in栈只进，out栈只出。out空了从In导入

```
class MyQueue {

    Stack<Integer> in, out;

    /** Initialize your data structure here. */
    public MyQueue() {
        in = new Stack<Integer>();
        out = new Stack<Integer>();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        in.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if(out.empty()){
            while(!in.empty()){
                out.push(in.pop());
            }
        }
        return out.pop();
    }

    /** Get the front element. */
    public int peek() {
        if(out.empty()){
            while(!in.empty()){
                out.push(in.pop());
            }
        }
        return out.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return in.empty() && out.empty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */
```