Implement Stack using Queues

Problem

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.
• int pop() Removes the element on the top of the stack and returns it.
• int top() Returns the element on the top of the stack.
• boolean empty() Returns true if the stack is empty, false otherwise.

Follow-up: Can you implement the stack using only one queue?

Constraints

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, top, and empty.
• All the calls to pop and top are valid.

Solution

The problem Implement Stack using Queues can be solved by using a queue to rotate previously pushed elements when a new push is called to simulate the order in which elements are stored in stack.

Implementation

class MyStack
{
  private:
    queue<int> _stack;

  public:
    MyStack() {}

    void push(int x)
    {
        _stack.push(x);
        for (int i = _stack.size(); i > 1; i--)
        {
            int front = _stack.front();
            _stack.pop();
            _stack.push(front);
        }
    }
    int pop()
    {
        int ret = _stack.front();
        _stack.pop();
        return ret;
    }

    int top() { return _stack.front(); }

    bool empty() { return _stack.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();
 */