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
class Solution {
public:
    vector<string> letterCasePermutation(string s) {
        string t = s;
        vector<string> ans;
        auto dfs = [&](this auto&& dfs, int i) -> void {
            if (i >= t.size()) {
                ans.push_back(t);
                return;
            }
            dfs(i + 1);
            if (isalpha(t[i])) {
                t[i] ^= 32;
                dfs(i + 1);
            }
        };
        dfs(0);
        return ans;
    }
};
