input: Write a python function to convert a given number to digital clock format.
  assert num2clock(3661) == '01:01:01'
  assert num2clock(59) == '00:00:59'
  assert num2clock(3600) == '01:00:00'
  assert num2clock(86399) == '23:59:59'
  assert num2clock(0) == '00:00:00'
output: def num2clock(seconds):
      hours = seconds // 3600
      minutes = (seconds % 3600) // 60
      seconds = seconds % 60
      return f"{hours:02}:{minutes:02}:{seconds:02}"