Depth First Search

def depth_first_search(start, goal, graph):
    """
    start is a str representation of a node in the graph
    goal is a str representation of a node in the graph
    graph is a dictionary of adjacency lists

    e.g. graph = {'A': ['B', 'C'], 'B': ['C', 'D'], 'C': ['D'], 'D': ['B', 'C']}
    """
    explored = set()
    stack = [[start]]
    viable_paths = []

    if start == goal:
        viable_paths.append([start])

    while stack:
        path = stack.pop()
        node = path[-1]

        if node not in explored:
            for neighbor in graph[node]:
                new_path = path[:]
                new_path.append(neighbor)

                if neighbor == goal:
                    viable_paths.append(new_path)
                else:
                    stack.append(new_path)

    return viable_paths