0784 Letter Case Permutation

0784 Letter Case Permutation.py

class Solution:
    def letterCasePermutation(self, S: str) -> List[str]:
        res = []
        def helper(a, pos):
            if pos == len(a):
                res.append(''.join(a))
            else:
                helper(a, pos + 1)
                if a[pos].isalpha():
                    a[pos] = a[pos].upper()
                    helper(a, pos + 1)
                    a[pos] = a[pos].lower()
        
        a = list(S.lower())
        helper(a, 0)
        return res