CareerCup

2 stacks: one for back_(pop/push) operations and the other one is for front_(pop/push) operations.

For example, if current operation is front operation (second stack involved) and the last was in back (i.e. on 1st stack) than just pop items from 1st and push to 2nd stack. And vice versa.

one stack
insert - push - O(1)
dequeue - pop - O(n)

Queue is First in First Out.
Stack is Last in First Out.
In order to mimic the Queue behavior, we will use two stacks.
Pop() all elements from StackA to StackB one by one - O(n)
Push() new element in StackA, so it stays at the bottom- O(1). Pop() all elements from StackB back to StackA, one by one so that the very first element in StackA would stay on the top- O(n).

int insertQ(int item)
{
   stack1.push(item);
   return;
}
int deleteQ(void)
{
  if(isempty(stack1)&&isempty(stack2))
      return UnderFlow; 
  if(!isempty(stack2))
      return pop(stack2);
   while(!isempty(stack1))
      stack2.push(stack1.pop());
   return pop(stack2);
}