Implement a stack using single queue

// C++ program to implement a stack using // single queue #include<bits/stdc++.h> using namespace std;   // User defined stack that uses a queue class Stack {     queue<int>q; public:     void push(int val);     void pop();     int top();     bool empty(); };   // Push operation void Stack::push(int val) {     //  Get previous size of queue     int s = q.size();       // Push current element     q.push(val);       // Pop (or Dequeue) all previous     // elements and put them after current     // element     for (int i=0; i<s; i++)     {         // this will add front element into         // rear of queue         q.push(q.front());           // this will delete front element         q.pop();     } }   // Removes the top element void Stack::pop() {     if (q.empty())         cout << "No elements\n";     else         q.pop(); }   // Returns top of stack int  Stack::top() {     return (q.empty())? -1 : q.front(); }   // Returns true if Stack is empty else false bool Stack::empty() {     return (q.empty()); }   // Driver code int main() {     Stack s;     s.push(10);     s.push(20);     cout << s.top() << endl;     s.pop();     s.push(30);     s.pop();     cout << s.top() << endl;     return 0; }