<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 {
    bool isalpha(char &a) {
        if (a >= 'a' && a <= 'z') {
            return true;
        } else if (a >= 'A' && a <= 'Z') {
            return true;
        }

        return false;
    }

    void toggle(char &a) {
        if (a >= 'a' && a <= 'z') {
            a =  a - 32;
        } else  {
            a = a + 32;
        }
    }

    void util(vector<string>&output, int index, string input) {
        if (index == input.length()) {
            output.push_back(input);
            return;
        }

        util(output, index + 1, input);
        
        if (isalpha(input[index])) {
            toggle(input[index]);
            util(output, index + 1, input);
            toggle(input[index]);
        }
    }
public:
    vector<string> letterCasePermutation(string s) {
        vector<string>output;

        util(output, 0, s);
        return output;
    }
};
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