# 232. Implement Queue using Stacks

[**232. Implement Queue using Stacks**](https://leetcode.com/problems/implement-queue-using-stacks/description/)

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.

**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).

**My Solutions:**

需要两个stack才能通过两次压栈变成queue的顺序

1。方法1

未经处理的stack是s，将s pop出再push进q，就变成了正确的q的顺序

pop, peek: O(n); 

push, empty: O(1)

```java
class MyQueue {

    Stack<Integer> s, q;
    
    /** Initialize your data structure here. */
    public MyQueue() {
        s = new Stack<>();
        q = new Stack<>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        s.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if (!q.isEmpty()) {
            return q.pop();
        } else {
            while (!s.isEmpty()) {
                q.push(s.pop());
            }
            return q.pop();
        }    
    }
    
    /** Get the front element. */
    public int peek() {
        if (!q.isEmpty()) {
            return q.peek();
        } else {
            while (!s.isEmpty()) {
                q.push(s.pop());
            }
            return q.peek();
        } 
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s.isEmpty() && q.isEmpty();
    }
}
```

2。 方法2

push: O(n);&#x20;

pop, peek, empty: O(1)

在push进去的时候，多一个temp，用来把stack里pop出的元素的再push回去，让stack的顺序变成queue的顺序

```
class MyQueue {

    Stack<Integer> q, temp;
    
    /** Initialize your data structure here. */
    public MyQueue() {
        temp = new Stack<>();
        q = new Stack<>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        if (q.isEmpty()) {
            q.push(x);
        }
        else {
            while (!q.isEmpty()) {
                temp.push(q.pop());
            }
            temp.push(x);
            while (!temp.isEmpty()) {
                q.push(temp.pop());
            }
        }
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        return q.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        return q.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return q.isEmpty();
    }
}
```