Recursion Practice(conti.)

# The Three Laws of Recursion 
# 1. A recursive algorithm must have a base case.
# 2. A recursive algorithm must change its state and move toward the base case.
# 3. A recursive algorithm must call itself, recursively.

# Reverse String
def reverse(s):
  if len(s) == 0 or len(s) == 1: # first law
    return s
  else:
    return s[len(s) - 1] + reverse(s[:-1]) # second, third law


# Palindrome
def removeWhite(s):
  newStr = ""
  for i in range(len(s)):
    if s[i].isalpha():
      newStr += s[i].lower()
  return newStr

def isPal(s):
  if len(s) == 1: # first law
    return True
  elif len(s) == 2: # first law
    return s[0] == s[1]
  else:
    if s[0] == s[-1]: # second and third law
      return isPal(s[1:-1]) 
    else:
      return False


def main():
  # Reverse String Test
  print(reverse("hello"))
  print(reverse("l"))
  print(reverse("follow"))
  print(reverse(""))
  
  # Palindrome Test
  print(isPal(removeWhite("I am a boy!")))
  
main()