Depth-first search (recursive)

"""
You'll see that the iterative version prints out 1-3-2-5-4, while the recursive version prints out 1-2-4-5-3.
What determines the different order of the handling of the nodes between the iterative and recursive versions of this code
is the fact that in the iterative version the 'for neighbor in graph' finishes adding all of the neighbors before the next
neighbor is popped off the stack, so the later neighbors get recursed before the earlier neighbors, whereas in the recursive
version the neighbors are recursed immediately as they're encountered within that for loop, so the earlier neighbors get
recursed before the later neighbors.
So, for example, if the graph says "1: [2, 3]", iterative DFS will have the 3 visited first, recursive DFS will have the 2 visited first.

Pseudocode:

# dfs_r - need graph, current_node, and visited
    # Initialize the set of visited nodes
    # Mark the current node as visited
    # Process the current node (for example, print it)
    # Loop through neighbors and recursively visit each
"""
def dfs_r(graph, start, visited=None):
    if visited is None:
        visited = set() # Initialize the set of visited nodes

    visited.add(start) # Mark the current node as visited
    print(start) # Process the current node (for example, print it)

    for neighbor in graph[start]:
        if neighbor not in visited:
            dfs_r(graph, neighbor, visited) # Recursively visit unvisited neighbors

graph = {
    1 : [2, 3],
    2 : [4, 5],
    3 : [],
    4 : [5],
    5 : [1],
}
dfs_r(graph, 1)  # Start from vertex 1