Letter Case Permutation

Given a string s, you can transform every letter individually to be lowercase or uppercase to create another string.

Return a list of all possible strings we could create. Return the output in any order.

Example 1:

Input: s = "a1b2"
Output: ["a1b2","a1B2","A1b2","A1B2"]

Example 2:

Input: s = "3z4"
Output: ["3z4","3Z4"]

Solution

class Solution:
    def letterCasePermutation(self, s: str) -> List[str]:
        out = list()

        def backtrack(sub, i):
            if len(sub) == len(s):
                out.append(sub)
            else:
                if s[i].isalpha():
                    backtrack(sub + s[i].swapcase(), i + 1)
                backtrack(sub + s[i], i + 1)

        backtrack('', 0)
        return out