Bus Routes
<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>
Breadth-First Search; Array; Hash Table
public class Solution {
    public int NumBusesToDestination(int[][] routes, int source, int target) {
        if (source == target) {
            return 0;
        }

        // Use Dictionary to map stops to bus routes
        var g = new Dictionary<int, List<int>>();
        for (int i = 0; i < routes.Length; i++) {
            foreach (int stop in routes[i]) {
                if (!g.ContainsKey(stop)) {
                    g[stop] = new List<int>();
                }
                g[stop].Add(i);
            }
        }

        // If source or target is not in the mapping, return -1
        if (!g.ContainsKey(source) || !g.ContainsKey(target)) {
            return -1;
        }

        // Initialize queue and visited sets
        var q = new Queue<int[]>();
        var visBus = new HashSet<int>();
        var visStop = new HashSet<int>();
        q.Enqueue(new int[] { source, 0 });
        visStop.Add(source);

        // Begin BFS
        while (q.Count > 0) {
            var current = q.Dequeue();
            int stop = current[0], busCount = current[1];

            // If the current stop is the target stop, return the bus count
            if (stop == target) {
                return busCount;
            }

            // Traverse all bus routes passing through the current stop
            foreach (int bus in g[stop]) {
                if (visBus.Add(bus)) {
                    // Traverse all stops on this bus route
                    foreach (int nextStop in routes[bus]) {
                        if (visStop.Add(nextStop)) {
                            q.Enqueue(new int[] { nextStop, busCount + 1 });
                        }
                    }
                }
            }
        }

        return -1; // If unable to reach the target stop, return -1
    }
}
