Implement Stack using Queues

Practice Link

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.

Approach — Costly Push#

To simulate LIFO with FIFO queues, make push expensive: enqueue the new element into q2, then move all existing elements from q1 into q2 so the new element ends up at the front. Swap q1 and q2. Now q1 holds elements in LIFO order — pop and top simply dequeue/peek from the front of q1 in O(1). The trade-off is O(n) push.

class MyStack {
public:
    queue<int> q1;
    queue<int> q2;
    MyStack() {
        
    }
    
    void push(int x) {
        q2.push(x);
        while(!empty())
        {
            q2.push(q1.front());
            q1.pop();
        }
        q1.push(x);
        q1 = q2;
        q2= queue<int>();
    }
    
    int pop() {
        if(!empty()){
            int temp = q1.front();
            q1.pop();
            return temp;
        }
        return -1;
    }
    
    int top() {
        return q1.front();
    }
    
    bool empty() {
        return q1.empty();
    }
};

Time Complexity:

• push: O(n)
• pop: O(1)

Space Complexity: O(n)