LeetCode [815] Bus Routes

815. Bus Routes

Hard

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.

 

Constraints:

• 1 <= routes.length <= 500.
• 1 <= routes[i].length <= 10^5.
• 0 <= routes[i][j] < 10 ^ 6.

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class Solution { public: int numBusesToDestination(vector<vector<int>>& routes, int S, int T) { if(S==T) return 0; set<int> visitedBus, visitedStops; int ret = 0; map<int, vector<int>> mp;//stop, bus for(int i=0; i<routes.size(); ++i) { for(auto st:routes[i]){ mp[st].push_back(i); } } queue<int> que;//stops que.push(S); queue<int> tmp; while(!que.empty()) { int curStop = que.front(); que.pop(); for(auto b:mp[curStop]){ if(visitedBus.count(b)) continue; visitedBus.insert(b); for(auto s:routes[b]){ if(visitedStops.count(s)) continue; if(s==T) return ret+1; visitedStops.insert(s); tmp.push(s); } } if(que.empty()){ ret++; que = tmp; queue<int> empty; swap(tmp, empty); } } return -1; } };