code: def dfs_traverse(graph, start):
      """
      Traversal by depth first search.
      """
      visited, stack = set(), [start]
      while stack:
          node = stack.pop()
          if node not in visited:
              visited.add(node)
              for next_node in graph[node]:
                  if next_node not in visited:
                      stack.append(next_node)
      return visited
  
label_desc: Use image node_id 1 for calling a global function with example usage: dfs_traverse(graph, start) and returns: visited