Implement Stack using Queues

 1 class Stack {
 2 private:
 3     queue <int> q_1;
 4     queue <int> q_2;
 5 public:
 6     // Push element x onto stack.
 7     void push(int x) {
 8         if(q_1.empty())
 9           q_1.push(x);
10         else
11         {
12             while(!q_1.empty())
13             {
14                 q_2.push(q_1.front());
15                 q_1.pop();
16             }
17             q_1.push(x);
18             while(!q_2.empty())
19             {
20                 q_1.push(q_2.front());
21                 q_2.pop();
22             }
23         }
24     }
25 
26     // Removes the element on top of the stack.
27     void pop() {
28         q_1.pop();
29     }
30 
31     // Get the top element.
32     int top() {
33         return q_1.front();
34     }
35 
36     // Return whether the stack is empty.
37     bool empty() {
38         return q_1.empty();
39     }
40 };