[leetcode]815. Bus Routes

链接：https://leetcode.com/problems/bus-routes/description/

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.

class Solution {
public:
    int numBusesToDestination(vector<vector<int>>& routes, int S, int T) {
        if(S==T)
            return 0;
        
        unordered_map<int,set<int>> m; // store stop-> route id set
        for(int i=0;i<routes.size();i++)
        {
            for(auto& stop : routes[i])
            {
                m[stop].insert(i);
            }
        }
        
        queue<int> q; // store route id
        set<int> seen;
        for(auto& r:m[S])
        {
            q.push(r);
            seen.insert(r);
        }
        
        int level=1;
        while(!q.empty())
        {
            int sz=q.size();
            for(int i=0;i<sz;i++)
            {
                auto& r=q.front();   //r denotes route id
                q.pop();
                
                for(auto& stop : routes[r])
                {
                    if(stop==T)
                        return level;
                    for(auto& nr:m[stop])  // nr denotes route id
                    {
                        if(seen.count(nr)==0)
                        {
                            q.push(nr);
                            seen.insert(nr);
                        }
                    }
                }
            }
            level++;
        }
        return -1;
    }
};