<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;

        std::map<int, unordered_set<int>> neighbours;

        for (int i = 0; i< routes.size(); i++)
        {
            //std::unordered_set<int> route(r.begin(), r.end());
            for (auto r : routes[i])
            {
                if (neighbours.contains(r))
                    neighbours[r].insert(i);
                else
                    neighbours[r] = {i};
            }
        }

        int dist = 1;
        if (!neighbours.contains(source) || !neighbours.contains(target))
            return -1;

        std::queue<int> qCurr;
        std::queue<int> qNext;
        std::unordered_set<int> seen;

        qCurr.push(source);

        while (!qCurr.empty())
        {
            int elem = qCurr.front();
            qCurr.pop();
            //cout << elem << endl;
            if (!seen.contains(elem))
            {
                seen.insert(elem);
                for ( auto n : neighbours[elem])
                {
                    for (auto r: routes[n])
                    {
                        if(!seen.contains(r))
                            qNext.push(r);
                        if (r == target)
                            return dist;
                    }
                }

            }

            if (qCurr.empty())
            {
                //cout << "swap" << endl;
                std::swap(qCurr, qNext);
                dist++;
            }
        }

        return -1;
    }
};
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