<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:
    const int N = 1e6+10;
    int numBusesToDestination(vector<vector<int>>& routes, int source, int target) {
        if (source == target) return 0;
        
        vector<vector<int>>g(N); // Routes associated with every stop
        vector<int>vis(routes.size()+10, 0); // Track of vis routes
        map<int, vector<int>>mp; // Extracting stops based on route no.

        int z = 1;
        for(auto it : routes){
            mp[z] = it;
            for(auto it1 : it){
                g[it1].push_back(z);
            }
            z++;
        }

        int step = 1;
        
        queue<int>q;
        
        for(auto it : g[source]){
            q.push(it); // Routes associated with source
            vis[it] = 1;
        }
        q.push(-1);

        while(!q.empty()){
            int route = q.front();
            q.pop();

            if (route == -1){
                step++;
                if (!q.empty()){
                    q.push(-1);
                }
                continue;
            }
            
            for(auto it : mp[route]){
                if (it == target){
                    return step;
                }
                for(auto it1 : g[it]){
                    if (!vis[it1]){
                        q.push(it1);
                        vis[it1] = 1;
                    }
                }
            }

        }

        return -1;
    }
};
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