<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;
        vector<int> st_vis(1000000,0);
        int n=routes.size(),i;
        vector<int> bus_vis(n,0);
        unordered_map<int,vector<int>> st_bus;
        i=0;
        for(auto bus:routes)
        {
            for(auto st:bus)
            st_bus[st].push_back(i);

            i++;
        }

        queue<int> q;
        for(auto bus:st_bus[source])
        {
            for(auto st:routes[bus])
            {
                st_vis[st]=1;
                q.push(st);
            }
            bus_vis[bus]=1;
        }
        q.push(-1);
        int ans=1;
        while(!q.empty())
        {
            if(q.front()==-1)
            {
                q.pop();
                ans++;
                if(q.size())
                q.push(-1);
                continue;
            }
            int st=q.front();
            if(st==target)
            break;
            for(auto bus:st_bus[st])
            {
                if(!bus_vis[bus])
                for(auto sts:routes[bus])
                if(!st_vis[sts]){
                q.push(sts);
                st_vis[sts]=ans;
                }

                bus_vis[bus]=1;
            }
            q.pop();
        }
        if(st_vis[target]==0)
        return -1;
        else
        return ans;
    }
};
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