<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();
        set<int>st[n];
        for(int i=0;i<n;i++)
        {
            for(int j=0;j<routes[i].size();j++)
            {
                int city=routes[i][j];
                st[i].insert(city);
            }
        }
        queue<int>q;
        vector<int>ans(n,INT_MAX);
        vector<bool>vis(n,false);
        for(int i=0;i<n;i++)
            if(st[i].find(source)!=st[i].end()){
                q.push(i);
                ans[i]=1;
            }
        while(!q.empty())
        {
            int tp=q.front();
            q.pop();
            if(vis[tp])continue;
            vis[tp]=true;
            for(int i=0;i<n;i++)
            {
                if(vis[i]||i==tp)continue;
                for(auto j:st[i])
                {
                    if(st[tp].find((j))!=st[tp].end()&&ans[i]>ans[tp]+1)
                    {
                        ans[i]=ans[tp]+1;
                        q.push(i);
                        break;
                    }  
                }
            }
        }
        int mn=INT_MAX;
        for(int i=0;i<n;i++)
        {
            if(st[i].find(target)!=st[i].end())mn=min(mn,ans[i]);
        }
        if(mn==INT_MAX)return -1;
        return mn;      
    }
};
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