Pathfinder

In [30]: def pathfinder(rules):
    ...:     rule_map = {}
    ...:     chars = set()
    ...:     for rule in rules:
    ...:         rule_map[rule[0]] = rule[2]
    ...:         chars.add(rule[0])
    ...:         chars.add(rule[2])
    ...:     for val in rule_map.values():
    ...:         chars.discard(val)
    ...:     start = chars.pop()
    ...:     strr = start
    ...:     key = start
    ...:     while True:
    ...:         if key in rule_map:
    ...:             strr += rule_map[key]
    ...:             key = rule_map[key]
    ...:         else:
    ...:             break
    ...:     return strr