input: Write a python function to encode a number into base-62 string format.
  assert encode_base_62(0) == ""
  assert encode_base_62(1) == "1"
  assert encode_base_62(61) == "z"
  assert encode_base_62(62) == "10"
  assert encode_base_62(12345) == "3d7"
output: def encode_base_62(num):
      if num == 0:
          return ""
      base62_chars = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
      base62_string = ""
      while num > 0:
          base62_string = base62_chars[num % 62] + base62_string
          num //= 62
      return base62_string