<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 help(int i,int n,set <string> &st,string &s)
    {
       if(i>n-1)
       {
        st.insert(s);
        return;
       }
       help(i+1,n,st,s);
       if(s[i]>=97 && s[i]<=122)
       {
        s[i]-=32;
       }
       else if(s[i]>=65 && s[i]<=90)
       {
        s[i]+=32;
       }
       help(i+1,n,st,s);
       if(s[i]>=65 && s[i]<=90)
       {
        s[i]+=32;
       }
       else if(s[i]>=97 && s[i]<=122)
       {
        s[i]-=32;
       }
    }

    vector<string> letterCasePermutation(string s) {
       set <string> st;
       int i=0;
       int n=s.length();
       help(i,n,st,s);
       vector<string> vec(st.begin(),st.end());
       return vec;
    }
};
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