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
class Solution {
    public int minMutation(String startGene, String endGene, String[] bank) {
        Deque<String> q = new ArrayDeque<>();
        q.offer(startGene);
        Set<String> vis = new HashSet<>();
        vis.add(startGene);
        int depth = 0;
        while (!q.isEmpty()) {
            for (int m = q.size(); m > 0; --m) {
                String gene = q.poll();
                if (gene.equals(endGene)) {
                    return depth;
                }
                for (String next : bank) {
                    int c = 2;
                    for (int k = 0; k < 8 && c > 0; ++k) {
                        if (gene.charAt(k) != next.charAt(k)) {
                            --c;
                        }
                    }
                    if (c > 0 && !vis.contains(next)) {
                        q.offer(next);
                        vis.add(next);
                    }
                }
            }
            ++depth;
        }
        return -1;
    }
}