class Solution:
  def findLetterCaseStringPermutations(self, text):
   
    def backtrack(word, result, seen, index):
      result.append(word[:])
      
      for i in range(index, len(word)): # loop through each character in the text
        char = word[i]
       
        if i not in seen and not char.isnumeric(): # if the character is not a number and have no been seen 
          seen.add(i)
          new_word = word[:i] + char.swapcase() + word[i + 1:]
          backtrack(new_word, result, seen, i) # recurse back 
        
          seen.remove(i)

    result = []
    backtrack(text, result, set(), 0)
    return result