Question

I was recently asked this interview question:

How to retrieve the first 5 items in the stack ?

Stack as you know is Last In First Out

Interviewer asked for algorithm/pseudocode with high performance for a stack with only 2 operations, Pop() and Push().

My trivial answer:

Stack S2;
foreach (item in stack S1)
{
  object item = S1.Pop();
  S2.push(item)
}

for (int i=0 ; i<5 ; i++)
 Printf(S2.Pop());

He told me that we have another solution with higher performance but I cannot find one.

Answer 1

Seems to have been a communication issue. To get the first five elements that were put onto the stack then I would do the following:

input: Stack S1;
Stack S2;
Stack S3;
Object[5] elementArray;
int elementIndex = 0;

foreach (item in Stack S1)
{
  elementArray[elementIndex] = item;
  elementIndex = ++elementIndex % 5 // modulus operation.
  S2.push(item); // only needed to restore S1 to its' original state.
}

elementIndex = ++elementIndex % 5 // advance to the 5th element from the bottom.

for (int index = 0; index < 5; ++index)
{
  S3.push(elementArray[elementIndex]);
  elementIndex = ++elementIndex % 5
}

foreach (item in Stack S2)
{
  S1.push(item);
}

End Result:

1. S1 is unchanged.
2. S2 is empty.
3. S3 contains the first five elements (I think of these as the bottom 5 elements) from stack S1 in inverse order.