225 Implement Stack using Queues 队列实现栈

Implement the following operations of a stack using queues.

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

详见：https://leetcode.com/problems/implement-stack-using-queues/description/

Java实现：

方法一：两个队列实现，始终保持一个队列为空即可

class MyStack {
    private Queue<Integer> queue1;
    private Queue<Integer> queue2;

    /** Initialize your data structure here. */
    public MyStack() {
        queue1=new LinkedList<Integer>();
        queue2=new LinkedList<Integer>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        if(queue1.isEmpty()){
            queue1.offer(x);
            while(!queue2.isEmpty()){
                queue1.offer(queue2.poll());
            }
        }else if(queue2.isEmpty()){
            queue2.offer(x);
            while(!queue1.isEmpty()){
                queue2.offer(queue1.poll());
            }
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        if(queue1.isEmpty()){
            return queue2.poll();
        }else{
            return queue1.poll();
        }
    }
    
    /** Get the top element. */
    public int top() {
        if(queue1.isEmpty()){
            return queue2.peek();
        }else{
            return queue1.peek();
        }
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue1.isEmpty()&&queue2.isEmpty();
    }
}

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

方法二：一个队列实现栈

class MyStack {
    private Queue<Integer> queue;

    /** Initialize your data structure here. */
    public MyStack() {
        queue=new LinkedList<Integer>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        queue.offer(x);
        for(int i=0;i<queue.size()-1;++i){
            queue.offer(queue.poll());
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return queue.poll();
    }
    
    /** Get the top element. */
    public int top() {
        return queue.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue.isEmpty();
    }
}

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

C++实现：

方法一：两个队列实现，始终保持一个队列为空即可

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        if(que1.empty())
        {
            que1.push(x);
            while(!que2.empty())
            {
                que1.push(que2.front());
                que2.pop();
            }
        }
        else if(que2.empty())
        {
            que2.push(x);
            while(!que1.empty())
            {
                que2.push(que1.front());
                que1.pop();
            }
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int val;
        if(que1.empty())
        {
            val=que2.front();
            que2.pop();
        }
        else if(que2.empty())
        {
            val=que1.front();
            que1.pop();
        }
        return val;
    }
    
    /** Get the top element. */
    int top() {
        if(que1.empty())
        {
            return que2.front();
        }
        else if(que2.empty())
        {
            return que1.front();
        }
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return que1.empty()&&que2.empty();
    }
private:
    queue<int> que1;
    queue<int> que2;
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * bool param_4 = obj.empty();
 */

方法二：一个队列实现栈

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        que.push(x);
        for(int i=0;i<que.size()-1;++i)
        {
            que.push(que.front());
            que.pop();
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int val=que.front();
        que.pop();
        return val;
    }
    
    /** Get the top element. */
    int top() {
        return que.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return que.empty();
    }
private:
    queue<int> que;
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * bool param_4 = obj.empty();
 */