input: Write a python function to find all the starting indices of a substring in a given string.
  assert find_substring("Hello World", "lo Wo") == [3]
  assert find_substring("banana", "ana") == [1, 3]
  assert find_substring("abcdef", "gh") == []
output: def find_substring(s, sub):
      indices = []
      sub_len = len(sub)
      for i in range(len(s) - sub_len + 1):
          if s[i:i + sub_len] == sub:
              indices.append(i)
      return indices