<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>

class Solution {
public:
    int numBusesToDestination(vector<vector<int>>& routes, int source, int target) {
        // bfs:
        // shortest path
        if(source == target) return 0;

        vector<unordered_set<int>> routeAdjList(routes.size());
        vector<unordered_set<int>> routeToStopMap(routes.size());

        unordered_map<int, unordered_set<int>> stopToRouteMap;

        for(int i=0; i<routes.size(); i++){
            for(int j=0; j<routes[i].size(); j++){
                for(auto it: stopToRouteMap[routes[i][j]]){
                    routeAdjList[i].insert(it);
                    routeAdjList[it].insert(i);
                }
                stopToRouteMap[routes[i][j]].insert(i);
                routeToStopMap[i].insert(routes[i][j]);
            }
        }

        // routeAdjList
        // 0: [[1,2,7], 
        // 1: [3,6,7]]

        // for(int i=0; i<routes.size(); i++){
        //     if(routeToStopMap[i].count(source) && routeToStopMap[i].count(target)) return 0;
        // }


        queue<pair<int, int>> q;

        vector<bool> visited(routes.size(), false);

        for(auto it: stopToRouteMap[source]){
            q.push({it, 1});
            visited[it] = true;
        }

        while(q.size()){
            auto [route, cost] = q.front();
            q.pop();

            if(routeToStopMap[route].count(target)) return cost;

            for(auto it: routeAdjList[route]){
                if(!visited[it]){
                    q.push({it, cost+1});
                    visited[it] = true;
                }
            }
        }

        return -1;
    }
};
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