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
impl Solution {
    pub fn letter_case_permutation(s: String) -> Vec<String> {
        fn dfs(i: usize, t: &mut Vec<char>, ans: &mut Vec<String>) {
            if i >= t.len() {
                ans.push(t.iter().collect());
                return;
            }
            dfs(i + 1, t, ans);
            if t[i].is_alphabetic() {
                t[i] = (t[i] as u8 ^ 32) as char;
                dfs(i + 1, t, ans);
            }
        }

        let mut t: Vec<char> = s.chars().collect();
        let mut ans = Vec::new();
        dfs(0, &mut t, &mut ans);
        ans
    }
}
