class Solution {

public:

int numBusesToDestination(vector<vector<int>>& routes, int S, int T)

{

if (S==T) return 0;

unordered_map<int,vector<int>>stop2bus;

for (int i=0; i<routes.size(); i++)

{

for (auto j: routes[i])

stop2bus[j].push_back(i);

}

unordered_set<int>visitedStop;

unordered_set<int>visitedBus;

queue<int>q;

q.push(S);

visitedStop.insert(S);

int step = -1;

while (!q.empty())

{

step += 1;

int len = q.size();

while (len--)

{

int curStop = q.front();

q.pop();

for (auto bus: stop2bus[curStop])

{

if (visitedBus.find(bus)!=visitedBus.end())

continue;

visitedBus.insert(bus);

for (auto nextStop: routes[bus])

{

if (visitedStop.find(nextStop)!=visitedStop.end())

continue;

if (nextStop==T)

return step+1;

q.push(nextStop);

visitedStop.insert(nextStop);

}

}

}

}

return -1;

}

};