Leetcode: Letter case permutation

The task is to print all case permutations of letters from strings that can include digits and letters. For example, a1b becomes a1b, a1B, A1b, A1B. The trick here is to realize that there are 2^n possible permutations where n is the number of characters, excluding digits. One possible solution is shown below. The main idea is to generate binary numbers and then use the 1 and 0 in their places to either make the character uppercase or keep it lowercase. For example a decimal number 2 is represented by b10 in binary notation. The number 3 is represented by b11, and so on. When left padding - meaning to write a prefix of 0, all possible combinations are found.

from typing import List

class Solution:
    def letterCasePermutation(self, s: str) -> List[str]:
      output = []
      s = s.lower()
      n = sum([1 if c.isalpha() else 0 for c in s])
      
      for i in range(2**n):
        m = f'{bin(i)[2:]:0>{n}}'
        t = ''
        counter = 0
        for c in s:
          if c.isalpha():
            t += c.upper() if m[counter] == '1' else c
            counter += 1
          else:
            t += c
        output.append(t)
          
      return output