<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:
    vector<string>result;
    void solve(string s,string op){
        if(s.length()==0){
            result.push_back(op);
            return;
        }
        if(isalpha(s[0])){
            string op1 = op;
            string op2 = op;
            op1.push_back(tolower(s[0]));
            op2.push_back(toupper(s[0]));
            s.erase(s.begin());
            solve(s,op1);
            solve(s,op2);
        }
        else{ // digit
            string op1 = op;
            op1.push_back(s[0]);
            s.erase(s.begin()+0);
            solve(s,op1); 
        }
    }
    vector<string> letterCasePermutation(string s) {
        string op="";
        solve(s,op);
        return result;
    }
};
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