<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>

class Solution {
public:
    void permutation(const string& s, int index, string current, vector<string>& result) {
        if (index == s.length()) {
            result.push_back(current);
            return;
        }

        char ch = s[index];
        if (isalpha(ch)) {
         
            permutation(s, index + 1, current + static_cast<char>(tolower(ch)), result);

            permutation(s, index + 1, current + static_cast<char>(toupper(ch)), result);
        } else {
            
            permutation(s, index + 1, current + ch, result);
        }
    }

    vector<string> letterCasePermutation(const string& s) {
        vector<string> result;
        permutation(s, 0, "", result);
        sort(result.begin(), result.end()); 
        return result;
    }
};
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