<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;
        }
        vector<unordered_set<int>> v_route_sets;
        for(auto r : routes){
            unordered_set<int> s;
            for(auto i : r){
                s.insert(i);
            }
            v_route_sets.push_back(s);
        }
        vector<int> visited(v_route_sets.size(), 0);
        queue<int> q;
        for(int i=0; i < v_route_sets.size(); ++i){
            if (v_route_sets[i].count(source) != 0){
                q.push(i);
            }
        }
        int ret = 1;
        while(!q.empty()){
            int size = q.size();
            for(int s =0; s < size; ++s){
                int curr_index = q.front();
                q.pop();
                auto curr_route = v_route_sets[curr_index];
                if (curr_route.count(target) != 0){
                    return ret;
                }
                for(int i=0; i < v_route_sets.size(); ++i){
                    if (i != curr_index && !visited[i]){
                        for(int stop : curr_route){
                            if (v_route_sets[i].count(stop) != 0){
                                q.push(i);
                                visited[i] = 1;
                                break;
                            }
                        }
                    }
                }
            }
            ret ++;
        }
        return -1;
    }
};
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