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
function numBusesToDestination(routes: number[][], source: number, target: number): number {
    if (source === target) {
        return 0;
    }

    const g: Map<number, number[]> = new Map();
    for (let i = 0; i < routes.length; i++) {
        for (const stop of routes[i]) {
            if (!g.has(stop)) {
                g.set(stop, []);
            }
            g.get(stop)!.push(i);
        }
    }

    if (!g.has(source) || !g.has(target)) {
        return -1;
    }

    const q: [number, number][] = [[source, 0]];
    const visBus: Set<number> = new Set();
    const visStop: Set<number> = new Set([source]);

    for (const [stop, busCount] of q) {
        if (stop === target) {
            return busCount;
        }
        for (const bus of g.get(stop)!) {
            if (!visBus.has(bus)) {
                visBus.add(bus);
                for (const nextStop of routes[bus]) {
                    if (!visStop.has(nextStop)) {
                        visStop.add(nextStop);
                        q.push([nextStop, busCount + 1]);
                    }
                }
            }
        }
    }
    return -1;
}
