<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;
        unordered_map<int, unordered_set<int>> stop_route, route_stops;
        for(int i = 0; i < routes.size(); i++){
            for(int stop : routes[i]){
                stop_route[stop].insert(i);
                route_stops[i].insert(stop);
            }
        }
        queue<pair<int, int>> stop_dist;
        stop_dist.push({source, 0});
        unordered_set<int> visited_stop;
        visited_stop.insert(source);
        while(!stop_dist.empty()){

            pair<int, int> qfront = stop_dist.front();
            int cstop = qfront.first;
            int dist = qfront.second;
            stop_dist.pop();

            for(auto it1 = stop_route[cstop].begin(); it1 != stop_route[cstop].end();){
                for(auto it2 = route_stops[(*it1)].begin(); it2 != route_stops[(*it1)].end();){
                    
                    if((*it2) == target)
                        return dist + 1;
                    stop_dist.push({*it2, dist + 1});
                    int stop = (*it2);
                    it2 = route_stops[(*it1)].erase(it2);
                    if(stop != cstop){
                        stop_route[stop].erase(*it1);
                    }
                }
                it1 = stop_route[cstop].erase(it1);
            }
        }
        return -1;
    }
};
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