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 = [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.
1 <= routes.length <= 500.
1 <= routes[i].length <= 500.
0 <= routes[i][j] < 10 ^ 6.
# Time: O(nlogn) # Space: O(n) class Solution: def numBusesToDestination(self, routes: List[List[int]], S: int, T: int) -> int: if S == T: return 0 stop_board = collections.defaultdict() for bus, stops in enumerate(routes): for stop in stops: if stop not in stop_board: stop_board[stop] = [bus] else: stop_board[stop].append(bus) queue = collections.deque([S]) visited = set() res = 0 while queue: res += 1 size_stops_reach = len(queue) for _ in range(size_stops_reach): cur_stop = queue.popleft() for bus in stop_board[cur_stop]: if bus in visited: continue visited.add(bus) for stop in routes[bus]: if stop == T: return res queue.append(stop) return -1 # 45/45 cases passed (392 ms) # Your runtime beats 92 % of python3 submissions # Your memory usage beats 100 % of python3 submissions (31.6 MB)