<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>

int bfs(vector<vector<int>> & g,vector<vector<int>> & routes, int s , int t, unordered_set<int> & vis){
    queue<int> q;
    for(auto bus: g[s]) {
        if(vis.find(bus) == vis.end()) {
            q.push(bus);
            vis.insert(bus);
        }
    }
    int ans = 1;
    if(s==t)
    return 0;

    while(!q.empty()) {
        int size = q.size();
        for(int i=0; i<size; i++) {
            int x = q.front();
            q.pop();

            for(auto stop: routes[x]){
                if(stop == t)
                    return ans;
                for(auto bus: g[stop]) {
                    if(vis.find(bus)==vis.end()) {
                        q.push(bus);
                        vis.insert(bus);
                    }
                }
            }
        }

        ans++;
    }
    return -1;

}

class Solution {
public:
    int numBusesToDestination(vector<vector<int>>& routes, int source, int target) {
        int n = routes.size();
        int m = 1000001;
        unordered_set<int> vis;
        vector<vector<int>> g(m, vector<int>());
        for(int i=0; i<n; i++) {
            for(auto x:routes[i]){
                g[x].push_back(i);
            }
        }
        int ans = 0;
        return bfs(g,routes, source, target, vis);
    }
};
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