# Adjacency list
first_graph = {
    1: [3, 20],
    2: [20],
    3: [1, 20],
    20: [2, 3]
}

# First exercise: go from 1, to 20, to 2
first_graph[first_graph[1][1]][0]

# pass visited by reference
def depth_first_search(graph: dict, start, visited=set()):
    print(start)
    if len(visited)==len(graph):
        return visited

    for x in graph[start]:
        if x not in visited:
            visited.add(x)
            return depth_first_search(graph, x, visited)
    pass
    

depth_first_search(first_graph, 1)