<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>>& busRoutes, int start, int end) {
        if (start == end) return 0;
        
        int maxBusStop = findMaxBusStop(busRoutes);
        if (maxBusStop < end || start > maxBusStop || end > maxBusStop) {
            return -1;
        }
        
        unordered_map<int, unordered_set<int>> stopToRoutes;
        for (int i = 0; i < busRoutes.size(); i++) {
            for (int stop : busRoutes[i]) {
                stopToRoutes[stop].insert(i);
            }
        }
        
        queue<pair<int, int>> q;
        q.push({start, 0});
        unordered_set<int> visitedStops;
        unordered_set<int> visitedRoutes;
        
        while (!q.empty()) {
            auto [currentStop, busesTaken] = q.front();
            q.pop();
            
            if (currentStop == end) {
                return busesTaken;
            }
            
            for (int routeIndex : stopToRoutes[currentStop]) {
                if (visitedRoutes.count(routeIndex)) continue;
                visitedRoutes.insert(routeIndex);
                
                for (int nextStop : busRoutes[routeIndex]) {
                    if (visitedStops.count(nextStop)) continue;
                    visitedStops.insert(nextStop);
                    q.push({nextStop, busesTaken + 1});
                }
            }
        }
        
        return -1;
    }
    
private:
    int findMaxBusStop(const vector<vector<int>>& routes) {
        int max = -1;
        for (const auto& route : routes) {
            for (int stop : route) {
                max = std::max(max, stop);
            }
        }
        return max;
    }
};
static const int kds = []() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    return 0;
}();

//kartikdevsharmaa
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