<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> letterCasePermutation(string s) {
        string perm = s;
        int n = s.size();
        vector<string> rst;

        auto swapLetter = [] (char& c) {
            if (c >= 'a' && c <= 'z') c = 'A' + (c - 'a');
            else c = 'a' + (c - 'A');
        };

        function<void(int)> dfs = [&] (int cur) {
            // if (cur >= n) return;
            rst.push_back(perm);
            for (int i = cur; i < n; i++) {
                if (s[i] >= '0' && s[i] <= '9') continue;
                swapLetter(perm[i]);
                dfs(i+1);
                swapLetter(perm[i]);
            }
        };
        dfs(0);
        return rst;
    }

};
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