Linked Structures

Insert at Head

Insert a new first node by pointing it at the old head and then moving the head pointer.

Algorithm

The replay labels nodes by value, such as node(20), and never exposes object identity or memory addresses. This Fortran DSA implementation uses the same small chain as the rest of the DSA track.

Basic Implementation

basic.f90 
program linked_list_lesson
    implicit none
    type :: list_node
        integer :: value
        type(list_node), pointer :: next => null()
    end type list_node
    type(list_node), pointer :: head, cursor, new_head
    type(list_node), pointer :: n1, n2, n3, n4

    allocate(n2); n2%value = 20
    allocate(n3); n3%value = 30
    n2%next => n3; n3%next => null()
    head => n2
    allocate(new_head); new_head%value = 10
    new_head%next => head
    head => new_head
    call print_chain(head)
contains
    subroutine print_chain(start)
        type(list_node), pointer :: start
        type(list_node), pointer :: cur
        cur => start
        do while (associated(cur))
            write(*, '(I0,A)', advance='no') cur%value, ' -> '
            cur => cur%next
        end do
        write(*, '(A)') 'null'
    end subroutine print_chain
end program linked_list_lesson

Complexity

  • Time: O(1)
  • Space: O(1)

Implementation notes

  • Keep the explicit node and pointer/reference operations; array shortcuts hide the linked-list state this lesson is meant to replay.
  • The final output prints the chain in a deterministic a -> b -> null form for cross-language comparison.
old head The previous first node becomes the second node.
constant-time insert Only the new node and head pointer change.