Letter Case Permutation
<p>Given a string <code>s</code>, you&nbsp;can transform every letter individually to be lowercase or uppercase to create another string.</p>

<p>Return <em>a list of all possible strings we could create</em>. Return the output in <strong>any order</strong>.</p>

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

<pre>
<strong>Input:</strong> s = &quot;a1b2&quot;
<strong>Output:</strong> [&quot;a1b2&quot;,&quot;a1B2&quot;,&quot;A1b2&quot;,&quot;A1B2&quot;]
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;3z4&quot;
<strong>Output:</strong> [&quot;3z4&quot;,&quot;3Z4&quot;]
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 12</code></li>
	<li><code>s</code> consists of lowercase English letters, uppercase English letters, and digits.</li>
</ul>
Bit Manipulation; String; Backtracking
function letterCasePermutation(s: string): string[] {
    const t = s.split('');
    const ans: string[] = [];
    const dfs = (i: number) => {
        if (i >= t.length) {
            ans.push(t.join(''));
            return;
        }
        dfs(i + 1);
        if (t[i].charCodeAt(0) >= 65) {
            t[i] = String.fromCharCode(t[i].charCodeAt(0) ^ 32);
            dfs(i + 1);
        }
    };
    dfs(0);
    return ans;
}
