<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 l{static_cast<int>(routes.size())};
    vector<vector<int>> edges(l);
    vector<unordered_set<int>> routeSets(l);
    vector<bool> hasTarget(l, false);
    vector<bool> visited(l, false);
    queue<int> q;

    for (int i = 0; i < l; i++) {
      bool hasSource{false};

      for (int n : routes[i]) {
        routeSets[i].insert(n);

        if (n == source)
          hasSource = true;

        if (n == target)
          hasTarget[i] = true;
      }

      if (hasSource && hasTarget[i])
        return 1;

      if (hasSource) {
        visited[i] = true;
        q.push(i);
      }
    }

    for (int i = 0; i < l; i++) {
      for (int j = i + 1; j < l; j++) {
        unordered_set<int>& smallSet{routeSets[i].size() <= routeSets[j].size()? routeSets[i] : routeSets[j]};
        unordered_set<int>& largeSet{routeSets[i].size() > routeSets[j].size()? routeSets[i] : routeSets[j]};
        bool connected{false};

        for (unordered_set<int>::iterator it = smallSet.begin(); it != smallSet.end() && !connected; it++) {
          if (largeSet.find(*it) != largeSet.end())
            connected = true;
        }

        if (connected) {
          edges[i].push_back(j);
          edges[j].push_back(i);
        }
      }
    }

    int output{2};

    while (!q.empty()) {
      int k{static_cast<int>(q.size())};

      for (int i = 0; i < k; i++) {
        for (int n : edges[q.front()]) {
          if (hasTarget[n])
            return output;

          if (!visited[n]) {
            visited[n] = true;
            q.push(n);
          }
        }

        q.pop();
      }

      output++;
    }

    return -1;
  }
};
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