<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>

#define ll long long
class Solution {
public:
    ll f(vector<int> &v1, vector<int> &v2) {
        ll i = 0, j=0;
        while(i<v1.size() && j<v2.size()) {
            if(v1[i]==v2[j]) return true;
            if(v1[i]<v2[j]) i++;
            else j++;
        }
        return false;
    }
    int numBusesToDestination(vector<vector<int>>& routes, int source, int target) {
        vector<vector<ll>> adj(501);

        ll n = routes.size();

        for(ll i=0; i<routes.size(); i++) sort(routes[i].begin(), routes[i].end());

        queue<pair<ll, ll>> q;

        for(ll i=0; i<n; i++) {
            for(ll j=0; j<routes[i].size(); j++) {
                if(routes[i][j]==source) {
                    q.push({i, 1});
                    break;
                }
            }
            for(ll j=i+1; j<n; j++) {
                if(f(routes[i], routes[j])) {
                    adj[i].push_back(j);
                    adj[j].push_back(i);
                }
            }
        }

        if(source==target) return 0;

        vector<ll> vis(n, 0);
        ll bc = 1;

        while(!q.empty()) {
            auto it = q.front();
            q.pop();

            ll bus = it.first, d = it.second;

            for(auto &i: routes[bus]) {
                if(i==target) return d;
            }

            for(auto &i: adj[bus]) {
                if(!vis[i]) {
                    vis[i]=1;
                    q.push({i, d+1});
                }
            }
        }

        return -1;
    }
};
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