We have a list of bus routes. Each routes[i] is a bus route that the i-th bus repeats forever. For example if routes[0] = [1, 5, 7], this means that the first bus (0-th indexed) travels in the sequence 1->5->7->1->5->7->1->... forever.
We start at bus stop S (initially not on a bus), and we want to go to bus stop T. Travelling by buses only, what is the least number of buses we must take to reach our destination? Return -1 if it is not possible.
Example:
Input: 
routes = [[1, 2, 7], [3, 6, 7]]
S = 1
T = 6
Output: 2
Explanation: 
The best strategy is take the first bus to the bus stop 7, then take the second bus to the bus stop 6.

 
Constraints:

1 <= routes.length <= 500.
1 <= routes[i].length <= 10^5.
0 <= routes[i][j] < 10 ^ 6.

```python

USER_FILENAME = "<user_code>"
import sys, json, builtins
import math, heapq, bisect, itertools, functools, collections, statistics, fractions, decimal, operator, array, random, re, string, os, pathlib
try:
    import resource as _res
except Exception:
    _res = None
from collections import Counter, defaultdict, deque, OrderedDict
from functools import lru_cache, reduce
from itertools import accumulate, permutations, combinations, product
from math import gcd, ceil, floor, sqrt
from decimal import Decimal
try:
    from typing import *
except Exception:
    pass
try:
    sys.setrecursionlimit(1_000_000)
except Exception:
    pass
try:
    import site as _site
    try:
        _sys_sites = _site.getsitepackages() if hasattr(_site, "getsitepackages") else []
        for _p in _sys_sites:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
    try:
        _usp = _site.getusersitepackages()
        if isinstance(_usp, str):
            _usp = [_usp]
        for _p in _usp:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
except Exception:
    pass

def _disabled_input(*args, **kwargs):
    raise RuntimeError("input() is disabled in callable mode")
builtins.input = _disabled_input

ns = {"__name__": "__judge__"}  # prevent if __name__ == "__main__" blocks
for _k, _v in list(globals().items()):
    if not _k.startswith("_"):
        ns[_k] = _v
_CODE = 'class Solution:\n    def numBusesToDestination(self, routes: List[List[int]], S: int, T: int) -> int:\n        # if S==T: return 0\n        # g = defaultdict(list)\n        # q = []\n        # t = set()\n        # routes = list(map(set, routes))\n        # for i, r1 in enumerate(routes):\n        #     if S in r1: q.append((i, 1))\n        #     if T in r1: t.add(i)\n        #     for j in range(i+1, len(routes)):\n        #         r2 = routes[j]\n        #         if any(s in r2 for s in r1):\n        #             g[i].append(j)\n        #             g[j].append(i)\n        # seen = set(q)\n        # while q:\n        #     node, jumps = q.pop(0)\n        #     if node in t: return jumps\n        #     for nxt in g[node]:\n        #         if nxt not in seen:\n        #             seen.add(nxt)\n        #             q.append((nxt, jumps+1))\n        # return -1\n        \n#         st = defaultdict(set)\n#         rt = defaultdict(set)\n#         for i, route in enumerate(routes):\n#             for stop in route:\n#                 st[stop].add(i)\n#                 rt[i].add(stop)\n        \n#         q = deque([(S,0)])\n#         st_seen = set()\n#         rt_seen = set()\n#         while q:\n#             node, jumps = q.popleft()\n#             if node == T: return jumps\n#             for r in st[node]:\n#                 if r not in rt_seen:\n#                     rt_seen.add(r)\n#                     for stop in routes[r]:\n#                         if stop not in st_seen:\n#                             q.append((stop, jumps+1))\n#                             st_seen.add(stop)\n#         return -1\n    \n    \n        to_routes = collections.defaultdict(set)\n        for i, route in enumerate(routes):\n            for j in route:\n                to_routes[j].add(i)\n        bfs = [(S, 0)]\n        seen = set([S])\n        for stop, bus in bfs:\n            if stop == T: return bus\n            for i in to_routes[stop]:\n                for j in routes[i]:\n                    if j not in seen:\n                        bfs.append((j, bus + 1))\n                        seen.add(j)\n                routes[i] = []  # seen route\n        return -1\n'
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'numBusesToDestination'
if _fn_name:
    if "Solution" in ns:
        try:
            _obj = ns["Solution"]()
            if hasattr(_obj, _fn_name):
                _target = getattr(_obj, _fn_name)
        except Exception:
            _target = None
    if _target is None and _fn_name in ns and callable(ns[_fn_name]):
        _target = ns[_fn_name]
# Fallback: if there's exactly one user-defined function, use it
if _target is None:
    _cands = [v for k, v in ns.items() if callable(v) and getattr(v, "__module__", "") in ("__judge__", "__main__")]
    _user_funcs = [c for c in _cands if getattr(c, "__name__", "").startswith("_") is False]
    if len(_user_funcs) == 1:
        _target = _user_funcs[0]
if _target is None:
    raise RuntimeError(f"Could not resolve callable '{_fn_name}'")

_raw = sys.stdin.read().strip()
if _raw:
    _parsed = json.loads(_raw)
    if isinstance(_parsed, dict):
        _kwargs = _parsed.get("kwargs", {})
        _args = _parsed.get("args", [])
    else:
        _args, _kwargs = _parsed, {}
else:
    _args, _kwargs = [], {}
if not isinstance(_args, (list, tuple)):
    _args = [_args]
_res = _target(*_args, **_kwargs)
try:
    sys.stdout.write(repr(_res) + "\n")
except Exception:
    pass

```

Measured memory_bytes: 7868