<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 m = routes.size(), f, g;
        vector<unordered_set<int>> graph(m);
        unordered_map<int, unordered_set<int>> rg;
        for(int i = 0; i < m; i++) {
            f = 0, g = 0;
            for(int j = 0; j < routes[i].size(); j++) {
                if(source == routes[i][j]) {
                    f = 1;
                }
                if(target == routes[i][j]) {
                    g = 1;
                }
                rg[routes[i][j]].insert(i);
            }
            if(f == 1 && g == 1) {
                return 1;
            }
        }
        int prev = -1;
        for(auto i : rg) {
            prev = -1;
            for(auto bus : i.second) {
                if(prev == -1) {
                    prev = bus;
                    continue;
                }
                // cout<<prev<<" "<<bus<<"\n";
                graph[prev].insert(bus);
                graph[bus].insert(prev);
            }
        }
        queue<vector<int>> q;
        vector<int> vis(m, 0);
        for(auto i : rg[source]) {
            q.push({i, 1});
            vis[i] = 1;
        }
        int curr, dist;
        while(!q.empty()) {
            curr = q.front()[0];
            dist = q.front()[1];
            q.pop();
            if(rg[target].find(curr) != rg[target].end()) {
                return dist;
            }
            for(auto i : graph[curr]) {
                // cout<<i<<" ";
                if(!vis[i]) {
                    q.push({i, dist + 1});
                    vis[i] = 1;
                }
            }
        }
        return -1;
    }
};
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