Stacks and Queues

Stack Push/Pop

Push values onto a stack and pop them back in last-in, first-out order.

Algorithm

The replay uses the same three values in every language, so this Fortran DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.f90 
program stack_queue_lesson
    implicit none
    integer :: values(3) = [10, 20, 30]
    integer :: result(3)
    integer :: i, count
    count = 0
    do i = 3, 1, -1
        count = count + 1
        result(count) = values(i)
    end do
    call print_values(result, count)
contains
    subroutine print_values(items, n)
        integer, intent(in) :: items(:)
        integer, intent(in) :: n
        integer :: j
        do j = 1, n
            if (j > 1) write(*, '(A)', advance='no') ' -> '
            write(*, '(I0)', advance='no') items(j)
        end do
        write(*, *)
    end subroutine print_values
end program stack_queue_lesson

Complexity

  • Time: O(1) per push/pop
  • Space: O(n)

Implementation notes

  • Keep the explicit stack/queue operations. Library shortcuts that only produce the final list hide the data-structure behavior this lesson is meant to replay.
  • The final output uses a deterministic a -> b -> c format for cross-language comparison.
top The top is the most recently pushed value.
LIFO A stack removes values in last-in, first-out order.