package main

import "fmt"

func main() {
    routes := [][]int{{1, 2, 7}, {3, 6, 7}}
    source := 1
    target := 6
    ret := numBusesToDestination(routes, source, target)
    fmt.Println(ret)
}

func numBusesToDestination(routes [][]int, source, target int) int {
    if source == target {
        return 0
    }
    n := len(routes)
    // key : 车站
    // value : list -> 该车站拥有哪些线路！
    map0 := make(map[int][]int)
    for i := 0; i < n; i++ {
        for j := 0; j < len(routes[i]); j++ {
            if _, ok := map0[routes[i][j]]; !ok {
                map0[routes[i][j]] = make([]int, 0)
            }
            map0[routes[i][j]] = append(map0[routes[i][j]], i)
        }
    }
    queue := make([]int, 0)
    set := make([]bool, n)
    for _, route := range map0[source] {
        queue = append(queue, route)
        set[route] = true
    }
    len0 := 1
    for len(queue) > 0 {
        nextLevel := make([]int, 0)
        for _, route := range queue {
            bus := routes[route]
            for _, station := range bus {
                if station == target {
                    return len0
                }
                for _, nextRoute := range map0[station] {
                    if !set[nextRoute] {
                        nextLevel = append(nextLevel, nextRoute)
                        set[nextRoute] = true
                    }
                }
            }
        }
        queue = nextLevel
        len0++
    }
    return -1
}