<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>

#define MAXI 1000001
#define vi vector<int>
#define vvi vector<vi>


class Solution {
public:
    int numBusesToDestination(vector<vector<int>>& routes, int source, int target) {
        
        vvi busStop(MAXI);
        int m = routes.size();
        for (int i=0; i<m; i++) 
            for (auto stop : routes[i]) busStop[stop].push_back(i);

        vi stopDist(MAXI, INT_MAX);
        vector<bool> routesDone(m, false);
        set<int> stopsDone;
        stopsDone.insert(source);
        stopDist[source] = 0;
        int cnt = 1;
        while (!stopsDone.empty()) {
            set<int> newStops;
            while (!stopsDone.empty()) {
                int currStop = *stopsDone.begin();
                stopsDone.erase(stopsDone.begin());
                for (auto buses : busStop[currStop]) 
                    if (routesDone[buses]) continue;
                    else {
                        routesDone[buses] = true;
                        for (auto nextStop : routes[buses])
                            if (stopDist[nextStop] != INT_MAX) continue;
                            else {
                                stopDist[nextStop] = cnt;
                                newStops.insert(nextStop);
                            }
                    }

            }
            stopsDone = newStops;
            cnt++;
        }
        return (stopDist[target] == INT_MAX) ? -1:stopDist[target];
    }
};
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