<p>You are given an array <code>routes</code> representing bus routes where <code>routes[i]</code> is a bus route that the <code>i<sup>th</sup></code> bus repeats forever.</p>

<ul>
	<li>For example, if <code>routes[0] = [1, 5, 7]</code>, this means that the <code>0<sup>th</sup></code> bus travels in the sequence <code>1 -&gt; 5 -&gt; 7 -&gt; 1 -&gt; 5 -&gt; 7 -&gt; 1 -&gt; ...</code> forever.</li>
</ul>

<p>You will start at the bus stop <code>source</code> (You are not on any bus initially), and you want to go to the bus stop <code>target</code>. You can travel between bus stops by buses only.</p>

<p>Return <em>the least number of buses you must take to travel from </em><code>source</code><em> to </em><code>target</code>. Return <code>-1</code> if it is not possible.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> routes = [[1,2,7],[3,6,7]], source = 1, target = 6
<strong>Output:</strong> 2
<strong>Explanation:</strong> The best strategy is take the first bus to the bus stop 7, then take the second bus to the bus stop 6.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> routes = [[7,12],[4,5,15],[6],[15,19],[9,12,13]], source = 15, target = 12
<strong>Output:</strong> -1
</pre>

<p>&nbsp;</p>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= routes.length &lt;= 500</code>.</li>
	<li><code>1 &lt;= routes[i].length &lt;= 10<sup>5</sup></code></li>
	<li>All the values of <code>routes[i]</code> are <strong>unique</strong>.</li>
	<li><code>sum(routes[i].length) &lt;= 10<sup>5</sup></code></li>
	<li><code>0 &lt;= routes[i][j] &lt; 10<sup>6</sup></code></li>
	<li><code>0 &lt;= source, target &lt; 10<sup>6</sup></code></li>
</ul>

bool init = []() {
   ios_base::sync_with_stdio(false);
   cin.tie(nullptr);
   cout.tie(nullptr);
   return true;
}();


class Solution {
    struct BusLine {
        int id;
        std::unordered_set<int> buses;
        bool intersects(const BusLine& rhs) const {
            for (int i : buses)
            {
                if (rhs.buses.contains(i))
                    return true;
            }
            return false;
        }
        // std::string toString() {
        //     std::ostringstream ostr;
        //     ostr << "BusLine [" << id << "] : (" << printContainer(buses) << ");";
        //     return ostr.str();
        // }
    };
public:
    int numBusesToDestination(vector<vector<int>>& routes, int source, int target) {
        if (source == target)
            return 0;
        std::unordered_map<int, BusLine> busLines;
        std::unordered_map<int, std::unordered_set<int>> busStopToBusLines;
        int numBusLines = 0;
        for (const auto& r : routes)
        {
            ++numBusLines;
            busLines[numBusLines].id = numBusLines;
            for (int i=0; i < r.size(); ++i)
            {
                busLines[numBusLines].buses.insert(r[i]);
                busStopToBusLines[r[i]].insert(numBusLines);
            }

        }

        // Construct adjacency list for our bus lines (which bus lines are connected to eachother)
        std::unordered_map<int, std::vector<int>> adjList;
        for (int i=1; i <= numBusLines; ++i)
        {
            for (int j=i+1; j <= numBusLines; ++j)
            {
                if (busLines[i].intersects(busLines[j]))
                {
                    adjList[i].push_back(j);
                    adjList[j].push_back(i);
                }
            }
        }

        // Now we bfs from our source station to our target station, and keep track of the number of different
        // buses that we have to take along our merry way.
        int numBuses = 0;
        std::unordered_set<int> visited;
        std::deque<int> todo;
        for (int tmp : busStopToBusLines[source])
        {
            todo.push_back(tmp);
            visited.insert(tmp);
        }

        while (!todo.empty())
        {
            ++numBuses;
            int sz = todo.size();
            while(sz--)
            {
                int busLine = todo.front();
                todo.pop_front();
                // Have we reached our target?
                if (busLines[busLine].buses.contains(target))
                    return numBuses;

                // How many other unvisited bus lines can we take?
                for (int nextBusLine : adjList[busLine])
                {
                    if (!visited.contains(nextBusLine))
                    {
                        todo.push_back(nextBusLine);
                        visited.insert(nextBusLine);
                    }
                }
            }
        }

        return -1;
    }
};
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