Queue using two Stacks.cpp

/*
Queue using two Stacks
======================

Implement a Queue using 2 stacks s1 and s2 .
A Query Q is of 2 Types
(i) 1 x (a query of this type means  pushing 'x' into the queue)
(ii) 2   (a query of this type means to pop element from queue and print the poped element)

Example 1:
Input:
5
1 2 1 3 2 1 4 2

Output: 
2 3

Explanation: 
In the first testcase
1 2 the queue will be {2}
1 3 the queue will be {2 3}
2   poped element will be 2 the queue 
    will be {3}
1 4 the queue will be {3 4}
2   poped element will be 3.

Example 2:
Input:
4
1 2 2 2 1 4

Output: 
2 -1

Explanation: 
In the second testcase 
1 2 the queue will be {2}
2   poped element will be 2 and 
    then the queue will be empty
2   the queue is empty and hence -1
1 4 the queue will be {4}.
Your Task:
You are required to complete the two methods push which take one argument an integer 'x' to be pushed into the queue and pop which returns a integer poped out from other queue(-1 if the queue is empty). The printing is done automatically by the driver code.

Expected Time Complexity : O(1) for push() and O(N) for pop() or O(N) for push() and O(1) for pop()  
Expected Auxilliary Space : O(1).

Constraints:
1 <= Q <= 100
1 <= x <= 100

Note:The Input/Ouput format and Example given are used for system's internal purpose, and should be used by a user for Expected Output only. As it is a function problem, hence a user should not read any input from stdin/console. The task is to complete the function specified, and not to write the full code.
*/

//Function to push an element in queue by using 2 stacks.
void StackQueue ::push(int x)
{
  s1.push(x);
}

//Function to pop an element from queue by using 2 stacks.
int StackQueue ::pop()
{
  if (s1.size() < 1)
    return -1;
  while (s1.size() > 1)
  {
    s2.push(s1.top());
    s1.pop();
  }
  int ans = s1.top();
  s1.pop();
  while (s2.size())
  {
    s1.push(s2.top());
    s2.pop();
  }
  return ans;
}