input: Write a python function to find all occurrences of a substring in a given string and return the concatenated result.
  assert find_substring("hellohello", "lo") == "lolo"
  assert find_substring("banana", "na") == "nana"
  assert find_substring("testcase", "t") == "tt"
output: def find_substring(s, sub):
      result = []
      start = 0
      while start < len(s):
          start = s.find(sub, start)
          if start == -1:
              break
          result.append(sub)
          start += len(sub)
      return ''.join(result)