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.

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 {
public:
    queue<int> q1, q2;
    // Push element x onto stack.
    void push(int x) {
        if ( q2.empty() ) q1.push(x);
        else q2.push(x);
        return;
    }

    // Removes the element on top of the stack.
    void pop() {
        if ( ! q2.empty() ) q1.swap(q2);
        while ( ! q1.empty() ) {
            int i = q1.front();
            q1.pop();
            if ( ! q1.empty() ) q2.push(i);
        }
        return;
    }

    // Get the top element.
    int top() {
        if ( ! q2.empty() ) q1.swap(q2);
        int i;
        while ( ! q1.empty() ) {
            i = q1.front();
            q1.pop();
            q2.push(i);
        }
        return i;
    }

    // Return whether the stack is empty.
    bool empty() {
        return (q1.empty() and q2.empty());
    }
};