Depth-First Search (Recursive)

Visit a start vertex, then recurse into its first unvisited neighbour all the way down before backtracking. A visited set prevents revisiting, and neighbour insertion order fixes the visit sequence.

Algorithm

On the canonical 6-vertex graph from graph-adjacency-list, starting at vertex 1, the deterministic visit order is 1 2 4 3 5 6 (space-separated). Calls unwind 6 -> 5 -> 4 -> 3 -> 2 -> 1 after all vertices are visited.

Basic Implementation

basic.f90

program graph_dfs
    implicit none
    integer, parameter :: N = 6
    integer :: adj(N, 3) = 0
    integer :: adj_len(N)
    integer :: visited(N)
    integer :: order(N)
    integer :: order_count, i

    adj_len = [2, 2, 2, 3, 2, 1]
    adj(1, 1:2) = [2, 3]
    adj(2, 1:2) = [1, 4]
    adj(3, 1:2) = [1, 4]
    adj(4, 1:3) = [2, 3, 5]
    adj(5, 1:2) = [4, 6]
    adj(6, 1:1) = [5]

    visited = 0
    order_count = 0

    call dfs(1)

    print '(*(I0,1X))', (order(i), i = 1, order_count)
contains
    recursive subroutine dfs(v)
        integer, intent(in) :: v
        integer :: k, nb
        visited(v) = 1
        order_count = order_count + 1
        order(order_count) = v
        do k = 1, adj_len(v)
            nb = adj(v, k)
            if (visited(nb) == 0) then
                call dfs(nb)
            end if
        end do
    end subroutine dfs
end program graph_dfs

Complexity

Time: O(V + E)
Space: O(V) recursion depth

Implementation notes

Fortran: a recursive subroutine dfs host-associates the adj, visited, and order arrays; the visit order prints space-separated.
The replay shows the current vertex, the visited set, and the running visit order after each entry, matching the lesson spec.

recursive descent Follow one branch to its end, then unwind and try the next neighbour.