# 225. Implement Stack using Queues

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.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

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).

# Solution

Approach 1: One queue or two queues.

# Code (Python)

Approach 1:

# Code (C++)

Approach 1:

class MyStack {
private:
    queue<int> q1;
    queue<int> q2;
public:
    /** Initialize your data structure here. */
    MyStack() {
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        // Two Queues, push - O(1), pop O(n).
        // One Queues, push - O(1), pop O(n).
        q1.push(x);
/*
        // Two Queues, push - O(n), pop O(1).
        q2.push(x);
        while (!q1.empty())
        {
            q2.push(q1.front());
            q1.pop();
        }
        queue<int> tmp = q1;
        q1 = q2;
        q2 = tmp;
*/
/*
        // One Queues, push - O(n), pop O(1).
        int q1Size = q1.size();
        q1.push(x);
        for (int i = 0; i< q1Size; ++i)
        {
            q1.push(q1.front());
            q1.pop();
        }
*/
    }

    /** Removes the element on top of the stack and returns that element. */
    int pop() {
/*
        // Two Queues, push - O(1), pop O(n).
        while (q1.size() > 1)
        {
            q2.push(q1.front());
            q1.pop();
        }
        int top = q1.front();
        q1.pop();
        queue<int> tmp = q1;
        q1 = q2;
        q2 = tmp;
        return top;
*/
        // One Queues, push - O(1), pop O(n).
        int q1Size = q1.size();
        for (int i = 0; i < q1Size - 1; ++i)
        {
            q1.push(q1.front());
            q1.pop();
        }
        int top = q1.front();
        q1.pop();
        return top;
/*        
        // Two Queues, push - O(n), pop O(1).
        // One Queues, push - O(n), pop O(1).
        int top = q1.front();
        q1.pop();
        return top;
*/
    }
    
    /** Get the top element. */
    int top() {
        // Two Queues, push - O(1), pop O(n).
        // One Queues, push - O(n), pop O(1).
        return q1.back();
/*
        // Two Queues, push - O(n), pop O(1).
        // One Queues, push - O(n), pop O(1).
        return q1.front();
*/
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q1.empty();
    }
};

/**
 * 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();
 */