import sys

def getQuadTree(str, index):
  index += 1
  if (str[index] == 'b'):
    return ('b', index)
  if (str[index] == 'w'):
    return ('w', index)

  (s1, index) = getQuadTree(str, index)
  (s2, index) = getQuadTree(str, index)
  (s3, index) = getQuadTree(str, index)
  (s4, indxe) = getQuadTree(str, index)

  return ('x'+s3+s4+s1+s2, index+1)

if __name__ == "__main__":
  case = int(sys.stdin.readline())

  for i in range(case):
    input_case = sys.stdin.readline()

    if len(input_case) == 2:
      print(input_case, end="")
    else:
      quadTree = getQuadTree(input_case, -1)
      print(quadTree[0])



이렇게 두 경우로 출력했는데요

import sys

def getQuadTree(str, index):
  index += 1
  if (str[index] == 'b'):
    return ('b', index)
  if (str[index] == 'w'):
    return ('w', index)

  (s1, index) = getQuadTree(str, index)
  (s2, index) = getQuadTree(str, index)
  (s3, index) = getQuadTree(str, index)
  (s4, indxe) = getQuadTree(str, index)

  return ('x'+s3+s4+s1+s2, index+1)

if __name__ == "__main__":
  case = int(sys.stdin.readline())
  total_case = []
  for i in range(case):
    input_case = sys.stdin.readline()
    total_case.append(input_case)

  for i in range(case):
    str = total_case[i]
    if len(str) == 2:
      print(str, end="")
    else:
      quadTree = getQuadTree(str, -1)
      print(quadTree[0])