Bus Routes

详细解题思路请访问：https://yanjia.me/zh/2018/11/…

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.

代码

func searchRoute2(routes [][]int, graph map[int]map[int]bool, src, dst int) int {
    queue := []int{}
    for routeNum := range graph[src] {
        queue = append(queue, routeNum)
    }
    visited := map[int]bool{}
    dstRoutes := map[int]bool{}
    // once one of the route in this map get hit, we find the solution
    for routeNum := range graph[dst] {
        dstRoutes[routeNum] = true
    }
    times := 1
    // start BFS
    for len(queue) != 0 {
        newQueue := []int{}
        for _, routeNum := range queue {
            if _, ok := dstRoutes[routeNum]; ok {
                return times
            }
            for _, stop := range routes[routeNum] {
                nextRouteNums := graph[stop]
                for nextRouteNum := range nextRouteNums {
                    // only add route that has been visited before to avoid cycle
                    if _, ok := visited[nextRouteNum]; !ok {
                        newQueue = append(newQueue, nextRouteNum)
                        visited[nextRouteNum] = true
                    }
                }
            }
        }
        queue = newQueue
        times++
    }
    return -1
}

// map bus stop number to bus route numbers
func buildGraph2(routes [][]int) map[int]map[int]bool {
    // use a map of map because route could be like 1->2->1->2
    graph := map[int]map[int]bool{}
    for i, route := range routes {
        for _, stop := range route {
            if _, ok := graph[stop]; ok {
                graph[stop][i] = true
            } else {
                graph[stop] = map[int]bool{
                    i: true,
                }
            }
        }
    }
    return graph
}

func numBusesToDestination(routes [][]int, S int, T int) int {
    if S == T {
        return 0
    }
    graph := buildGraph2(routes)
    return searchRoute2(routes, graph, S, T)
}