<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:
    set<string> st;

    void backtrack(string &s,int &n,int start){
        if(start==n){
            st.insert(s);
            return;
        }
        for(int i =start;i<n;i++){
            if(isdigit(s[i])){
                backtrack(s,n,i+1);
            }
            else{
                s[i] = toupper(s[i]);
                backtrack(s,n,i+1);
                s[i] = tolower(s[i]);
                backtrack(s,n,i+1);
            }
        }
    }

    vector<string> letterCasePermutation(string s) {
        int n = s.size();
        backtrack(s,n,0);
        return vector<string>(st.begin(),st.end());
    }
};
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