<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) {
        if(source == target) return 0;
        int n = routes.size();
        vector<vector<int> > m(1e6);
        vector<vector<bool> > adj(n+2, vector<bool> (n+2, 0));
        for(int i = 0; i < n; i++){
            for(int j = 0; j < routes[i].size(); j++){
                for(int k = 0; k < m[routes[i][j]].size(); k++){
                    adj[i][m[routes[i][j]][k]] = 1;
                    adj[m[routes[i][j]][k]][i] = 1;
                }
                m[routes[i][j]].push_back(i);
            }
        }
        if(!m[source].size() || !m[target].size()) return -1;
        for(int i = 0; i < m[source].size(); i++){
            adj[m[source][i]][n] = 1;
            adj[n][m[source][i]] = 1;
        }
        for(int i = 0; i < m[target].size(); i++){
            adj[m[target][i]][n+1] = 1;
            adj[n+1][m[target][i]] = 1;
        }
        priority_queue<int, vector<int>, greater<int> > q;
        vector<int> dist(n+2, 1e6);
        dist[n] = 0;
        q.push(n);
        while(!q.empty()){
            int k = q.size();
            for(int i = 0; i < k; i++){
                int t = q.top(); q.pop();
                for(int j = 0; j < n+2; j++){
                    if(adj[t][j]){
                        if(dist[j] > dist[t]+1){
                            dist[j] = dist[t]+1;
                            q.push(j);
                        }
                    }
                }
            }
        }
        if(dist[n+1] == 1e6) return -1;
        return dist[n+1]-1;
        /*
        vector<bool> mp(n+2, 1);
        queue<int> q;
        mp[n] = 0;
        q.push(n);
        int ans = 0;
        while(!q.empty()){
            ans++;
            int k = q.size();
            for(int i = 0; i < k; i++){
                int t = q.front(); q.pop();
                for(int j = 0; j < adj[t].size(); j++){
                    if(adj[t][j] == n+1) return ans-1;
                    if(mp[adj[t][j]]){
                        mp[adj[t][j]] = 0;
                        q.push(adj[t][j]);
                    }
                }
            }
        }
        return -1;
        */
    }
};
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