Add pre_order_dfs_vertices

Author: LinjianMa

src/Graphs/abstractgraph.jl

@@ -261,6 +261,11 @@ end
   return post_order_dfs_vertices(dfs_tree_graph, root_vertex)
 end
 
+@traitfn function pre_order_dfs_vertices(graph::::(!IsDirected), root_vertex)
+  dfs_tree_graph = dfs_tree(graph, root_vertex)
+  return pre_order_dfs_vertices(dfs_tree_graph, root_vertex)
+end
+
 @traitfn function post_order_dfs_edges(graph::::(!IsDirected), root_vertex)
   dfs_tree_graph = dfs_tree(graph, root_vertex)
   return post_order_dfs_edges(dfs_tree_graph, root_vertex)
@@ -336,6 +341,10 @@ end
   return [node.vertex for node in PostOrderDFS(TreeGraph(graph, root_vertex))]
 end
 
+@traitfn function pre_order_dfs_vertices(graph::::IsDirected, root_vertex)
+  return [node.vertex for node in PreOrderDFS(TreeGraph(graph, root_vertex))]
+end
+
 # Traverse the tree using a [post-order depth-first search](https://en.wikipedia.org/wiki/Tree_traversal#Depth-first_search), returning the edges where the source is the current vertex and the destination is the parent vertex.
 # Assumes the graph is a [rooted directed tree](https://en.wikipedia.org/wiki/Tree_(graph_theory)#Rooted_tree).
 # Returns a list of edges directed **towards the root vertex**!

src/NamedGraphs.jl

@@ -168,6 +168,7 @@ export NamedGraph,
   path_digraph,
   path_graph,
   periphery,
+  pre_order_dfs_vertices,
   post_order_dfs_vertices,
   post_order_dfs_edges,
   radius,