input: Write a function to find the start index of a substring in a given string. If the substring is not present in the string, return -1. If the given string is empty, return -1.
  def find_substring(source, substring):
      if substring not in source:
          return -1
      i=0
      while i < len(source):
          if source[i] == substring[0]:
              flag = True
              for j in range(len(substring)):
                  if substring[j] != source[i+j]:
                      flag = False
                      break
              if flag:
                  return i
          i += 1
      return -1
  
  assert find_substring("hello world", "world")==6
  assert find_substring("hello world", "hello")==0
  assert find_substring("hello world", "hello world")==0
  assert find_substring("hello world", "python")==-1
  assert find_substring("", "hello")==-1
output: def find_substring(source, substring):
      if not source or substring not in source:
          return -1
      return source.index(substring)