# 232.Implement Queue Using Stack

**232.Implement Queue Using Stacks**

难度:Easy

> 使用栈实现队列的下列操作：

push(x) -- 将一个元素放入队列的尾部。 pop() -- 从队列首部移除元素。 peek() -- 返回队列首部的元素。 empty() -- 返回队列是否为空。

```
示例:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // 返回 1
queue.pop();   // 返回 1
queue.empty(); // 返回 false
```

说明:

你只能使用标准的栈操作 -- 也就是只有 push to top, peek/pop from top, size, 和 is empty 操作是合法的。 你所使用的语言也许不支持栈。你可以使用 list 或者 deque（双端队列）来模拟一个栈，只要是标准的栈操作即可。 假设所有操作都是有效的 （例如，一个空的队列不会调用 pop 或者 peek 操作）。

队列的特性是FIFO，栈的特性是LIFO，两个栈就可实现FIFO。

```
class MyQueue {
private:
        stack<int> mq1;
        stack<int> mq2;
public:
    /** Initialize your data structure here. */
    MyQueue() {

    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        if(mq2.empty())
            mq2.push(x);
        else
        {
            while(!mq2.empty())
            {
                mq1.push(mq2.top());
                mq2.pop();
            }
            mq2.push(x);
            while(!mq1.empty())
            {
                mq2.push(mq1.top());
                mq1.pop();
            }
        }
   
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int t=mq2.top();
        mq2.pop();
        return t;

        
    }
    
    /** Get the front element. */
    int peek() {
        return mq2.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return mq2.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();
 * bool param_4 = obj->empty();
 */
```


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