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.

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:

class Stack {
private:
    queue<int> que1;
    queue<int> que2;
    void transfer(queue<int>& q1, queue<int>& q2) {
        while (q1.size() != 1) {
                q2.push(q1.front());
                q1.pop();
        }
    }
public:
    // Push element x onto stack.
    void push(int x) {
        if (que2.empty()) {
            que1.push(x);
        } else {
            que2.push(x);
        }
    }

    // Removes the element on top of the stack.
    void pop() {
        if (que1.empty()) {
            transfer(que2, que1);
            que2.pop();
        } else {
            transfer(que1, que2);
            que1.pop();
        }
    }

    // Get the top element.
    int top() {
        int ret;
        if (que1.empty()) {
            transfer(que2, que1);
            ret = que2.front();
            que2.pop();
            que1.push(ret);
        } else {
            transfer(que1, que2);
            ret = que1.front();
            que1.pop();
            que2.push(ret);
        }
        return ret;
    }

    // Return whether the stack is empty.
    bool empty() {
        return que1.empty() && que2.empty();
    }
};



====== Another way Clean and nice =====

class Stack {
public:
    // Push element x onto stack.
    void push(int x) {
        queue<int> tmp;
        while (!q.empty()) {
            tmp.push(q.front());
            q.pop();
        }
        q.push(x);
        while (!tmp.empty()) {
            q.push(tmp.front());
            tmp.pop();
        }
    }

    // Removes the element on top of the stack.
    void pop() {
        q.pop();
    }

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

    // Return whether the stack is empty.
    bool empty() {
        return q.empty();
    }
    
private:
    queue<int> q;
};