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[leetcode] problem 815 – bus routes

admin 11月 13, 2020 0

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.

Note

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

Code

public int (int[][] routes, int S, int T) {
    if (S == T)
        return 0;

    int result = 0;
    Map<Integer, List<Integer>> buses = new HashMap<>();
    Set<Integer> visit = new HashSet<>();
    Queue<Integer> queue = new LinkedList<>();
    queue.offer(S);

    for (int i = 0; i < routes.length; i++) {
        for (int stop : routes[i]) {
            buses.putIfAbsent(stop, new ArrayList<>());
            buses.get(stop).add(i);
        }
    }

    while (!queue.isEmpty()) {
        int size = queue.size();
        result++;

        for (int i = 0; i < size; i++) {
            int currentStop = queue.poll();

            for (int bus : buses.get(currentStop)) {
                if (visit.contains(bus))
                    continue;

                visit.add(bus);

                for (int stop : routes[bus]) {
                    if (stop == T)
                        return result;

                    queue.offer(stop);
                }
            }
        }
    }

    return -1;
}