Recursion strDist

# Given a string and a non-empty substring sub, compute recursively the
# largest substring which starts and ends with sub and return its length.
# str: 'foo', sub: 'foo' -> 3
# str: 'foo', sub: 'foobar' -> 0
# str: 'foobarfoo', sub: 'foo' -> 9
# str: 'foobarfooabc', sub: 'foo' -> 9
# str: 'abcfoobarfoo', sub: 'foo' -> 9
# str: 'abcfoobarfooabc', sub: 'foo' -> 9
def strDist(str, sub):
  # Get length of str
  str_len = len(str)
  # Get length of substring
  sub_len = len(sub)

  # If str len < sub len:
  if str_len < sub_len:
    # return 0
    return 0

  # If the 1st sub len chars of str are equal to sub
  if str[:sub_len] == sub:
    # If the last sub len chars of str are equal to sub
    if str[-sub_len:] == sub:
      # return str len
      return str_len

    # Else if the last sub len chars of str are NOT equal to sub
    else:
      # return strDist of str[:-1]
      return strDist(str[:-1], sub)

  # Else if 1st sub len chars of str are NOT equal to sub
  else:
    # If the last sub len chars of str are equal to sub
    if str[-sub_len:] == sub:
      # return strDist of str[1:]
      return strDist(str[1:], sub)

    # Else if last sub len chars of str are NOT equal to sub
    else:
      # return strDist of str[1:-1]
      return strDist(str[1:-1], sub)