Minimum Genetic Mutation
<p>A gene string can be represented by an 8-character long string, with choices from <code>&#39;A&#39;</code>, <code>&#39;C&#39;</code>, <code>&#39;G&#39;</code>, and <code>&#39;T&#39;</code>.</p>

<p>Suppose we need to investigate a mutation from a gene string <code>startGene</code> to a gene string <code>endGene</code> where one mutation is defined as one single character changed in the gene string.</p>

<ul>
	<li>For example, <code>&quot;AACCGGTT&quot; --&gt; &quot;AACCGGTA&quot;</code> is one mutation.</li>
</ul>

<p>There is also a gene bank <code>bank</code> that records all the valid gene mutations. A gene must be in <code>bank</code> to make it a valid gene string.</p>

<p>Given the two gene strings <code>startGene</code> and <code>endGene</code> and the gene bank <code>bank</code>, return <em>the minimum number of mutations needed to mutate from </em><code>startGene</code><em> to </em><code>endGene</code>. If there is no such a mutation, return <code>-1</code>.</p>

<p>Note that the starting point is assumed to be valid, so it might not be included in the bank.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> startGene = &quot;AACCGGTT&quot;, endGene = &quot;AACCGGTA&quot;, bank = [&quot;AACCGGTA&quot;]
<strong>Output:</strong> 1
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> startGene = &quot;AACCGGTT&quot;, endGene = &quot;AAACGGTA&quot;, bank = [&quot;AACCGGTA&quot;,&quot;AACCGCTA&quot;,&quot;AAACGGTA&quot;]
<strong>Output:</strong> 2
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= bank.length &lt;= 10</code></li>
	<li><code>startGene.length == endGene.length == bank[i].length == 8</code></li>
	<li><code>startGene</code>, <code>endGene</code>, and <code>bank[i]</code> consist of only the characters <code>[&#39;A&#39;, &#39;C&#39;, &#39;G&#39;, &#39;T&#39;]</code>.</li>
</ul>
Breadth-First Search; Hash Table; String
func minMutation(startGene string, endGene string, bank []string) int {
	type pair struct {
		s     string
		depth int
	}
	q := []pair{pair{startGene, 0}}
	vis := map[string]bool{startGene: true}
	for len(q) > 0 {
		p := q[0]
		q = q[1:]
		if p.s == endGene {
			return p.depth
		}
		for _, next := range bank {
			diff := 0
			for i := 0; i < len(startGene); i++ {
				if p.s[i] != next[i] {
					diff++
				}
			}
			if diff == 1 && !vis[next] {
				vis[next] = true
				q = append(q, pair{next, p.depth + 1})
			}
		}
	}
	return -1
}