<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>

auto _ = []() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  std::cout.tie(nullptr);
  return 0;
}();
class Solution {
  static auto dfs(std::vector<std::string>& res, std::string& s, std::size_t i) {
    if (i == s.size()) {
      res.push_back(s);
      return;
    }
    if (std::isalpha(s[i])) {
      s[i] = std::tolower(s[i]);
      dfs(res, s, i + 1);
      s[i] = std::toupper(s[i]);
    }
    dfs(res, s, i + 1);
  };

 public:
  std::vector<std::string> letterCasePermutation(std::string s) {
    auto res = std::vector<std::string>{};
    dfs(res, s, 0);
    return res;
  }
};
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