Implement Stacks using Queue

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).

Solution: Using only one queue

When we push an element into a queue, it will be stored at back of the queue due to queue's properties. But we need to implement a stack, where last inserted element should be in the front of the queue, not at the back. To achieve this we can invert the order of queue elements when pushing a new element.

class MyStack {
public:
    /** 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 val = q.front();
        q.pop();
        return val;
    }

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

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

private:
    queue<int> q;
};