Implement Stack using Two Queues

Question

A similar question was asked earlier there, but the question here is the reverse of it, using two queues as a stack. The question...

Given two queues with their sta

Answer 1

Let S1 and S2 be the two Stacks to be used in the implementation of queues.

struct Stack 
{ struct Queue *Q1;
  struct Queue *Q2;
}

We make sure that one queue is empty always.

Push operation : Whichever queue is not empty, insert the element in it.

• Check whether queue Q1 is empty or not. If Q1 is empty then Enqueue the element in it.
• Otherwise EnQueue the element into Q1.

Push (struct Stack *S, int data) { if(isEmptyQueue(S->Q1) EnQueue(S->Q2, data); else EnQueue(S->Q1, data); }

Time Complexity: O(1)

Pop Operation: Transfer n-1 elements to other queue and delete last from queue for performing pop operation.

• If queue Q1 is not empty then transfer n-1 elements from Q1 to Q2 and then, DeQueue the last element of Q1 and return it.
• If queue Q2 is not empty then transfer n-1 elements from Q2 to Q1 and then, DeQueue the last element of Q2 and return it.

`

int Pop(struct Stack *S){
int i, size;
if(IsEmptyQueue(S->Q2)) 
{
size=size(S->Q1);
i=0;
while(iQ2, Dequeue(S->Q1)) ;
  i++;
}
return DeQueue(S->Q1);  
}
else{
size=size(S->Q2);
while(iQ1, Dequeue(S->Q2)) ;
i++;
}
return DeQueue(S->Q2);
} }

Time Complexity: Running Time of pop Operation is O(n) as each time pop is called, we are transferring all the elements from one queue to oter.