Because this mission does not accept solutions, or allow you to publish them, I thought I would post my solution and see what you think.

def flatten(dictionary):
stack = [((), dictionary)]
result = {}
while stack:

    path, current = stack.pop()
    #print(current)
    #print(current.items())
    for k, v in current.items():
        if isinstance(v, dict):

            if v == {}:
                v = ""
                stack.append((path, {k : v}))
                continue
            stack.append((path + (k,), v))
        else:
            #print(path, k, v)
            result["/".join((path + (k,)))] = v
            #print(result)
return result

Thanks!