Queue from Two Stacks

Implement queue behavior with an input stack and an output stack.

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 = 1, 3
        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) amortized per operation
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.

input stack Enqueue pushes new values onto the input stack.

output stack When the output stack is empty, transferring all input values reverses them into dequeue order.