<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:

    def flip_case(self, c):
        if c.islower():
            return c.upper()
        return c.lower()

    def letterCasePermutation(self, s: str) -> List[str]:
        
        res = [s]
        # loop over chars in string
        for i in range(len(s)):
            char = s[i]
            if char.isalpha() is False:
                continue

            # loop over list
            limit = len(res)
            for j in range(limit):
                # convert word to list
                copy = list(res[j])
                flip_char = self.flip_case(char)
                copy[i] = flip_char
                res.append(''.join(copy))
        return res

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