The 'subtraction algorithm' should not be taught - it is not hard to invert the addition algorithm once learned (it's also more fun).

Example: 871 - 284 = ???

  284
+ ???
  ---
  871

   1
  284
+ ??7
  ---
  871

  11
  284
+ ?87
  ---
  871

  11
  284
+ 587
  ---
  871

, so ??? = 587

In what I think of as the subtraction algorithm, one proceeds from left to
right, which is confusing.