Implement Stack using Queues

Desicription

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:

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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

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/**
 * 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();
 */
class MyStack {
public:
    queue<int> q;
    /** Initialize your data structure here. */
    MyStack() {
    }

    /** Push element x onto stack. */
    void push(int x) {
        q.push(x);
        for(int i = 1; i < q.size(); i++) {
            q.push(q.front());
            q.pop();
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int result = q.front();
        q.pop();
        return result;
    }

    /** Get the top element. */
    int top() {
        return q.front();
    }

    /** Returns whether the stack is empty. */
    bool empty() {
        return q.empty();
    }
};