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
func numBusesToDestination(routes [][]int, source int, target int) int {
	if source == target {
		return 0
	}

	g := make(map[int][]int)
	for i, route := range routes {
		for _, stop := range route {
			g[stop] = append(g[stop], i)
		}
	}

	if g[source] == nil || g[target] == nil {
		return -1
	}

	q := list.New()
	q.PushBack([2]int{source, 0})
	visBus := make(map[int]bool)
	visStop := make(map[int]bool)
	visStop[source] = true

	for q.Len() > 0 {
		front := q.Front()
		q.Remove(front)
		stop, busCount := front.Value.([2]int)[0], front.Value.([2]int)[1]

		if stop == target {
			return busCount
		}

		for _, bus := range g[stop] {
			if !visBus[bus] {
				visBus[bus] = true
				for _, nextStop := range routes[bus] {
					if !visStop[nextStop] {
						visStop[nextStop] = true
						q.PushBack([2]int{nextStop, busCount + 1})
					}
				}
			}
		}
	}

	return -1
}
