<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 n_routes = routes.size(), ans = 0;
        vector<unordered_set<int>> routes_with_stop(1e6 + 1);
        vector<bool> seen_route(n_routes, false);
        queue<int> pending;
        for (int i = 0; i < n_routes; i++) {
            bool source_found = false, target_found = false;
            for (auto &stop : routes[i]) {
                routes_with_stop[stop].insert(i);
                if (stop == source) {
                    source_found = true;
                    pending.push(i);
                    seen_route[i] = true;
                } else if (stop == target) {
                    target_found = true;
                }
                if (source_found && target_found) {
                    return 1;
                }
            }
        }
        while (!pending.empty()) {
            ans++;
            for (int i = pending.size() - 1; i >= 0; i--) {
                int current_route = pending.front();
                pending.pop();
                for (auto &stop : routes[current_route]) {
                    if (stop == target) {return ans;}
                    for (auto &route : routes_with_stop[stop]) {
                        if (routes_with_stop[target].count(route) == 1) {
                            return ans + 1;
                        }
                        if (!seen_route[route]) {
                            pending.push(route);
                            seen_route[route] = true;
                        }
                    }
                }
            }
        }
        return -1;
    }
};
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