Implement C=|A-B| with inc,dec,jnz (A,B are non-negative)

Question

This is a question I saw in an interview :

A,B are non-negative numbers and you need to return C=|A-B| where you have only the following instructions:

Answer 1

A solution with just the instructions allowed may be this (though not elegant). Where the pseudo registers a and b holds the operands and the pseudo register c the result (which is initially zero as stated).

_dec_a:
 dec a
 inc a
jnz _dec_b

;a is zero here

  _a_zero_dec_b:
   dec b
   inc b
  jnz _a_zero_b_non_zero

;a and b are zero here 

   ;;C is the result
   inc c
   jnz _result

 _a_zero_b_non_zero:
   dec b
   inc c
 jnz _a_zero_dec_b

   ;CANNOT FALL HERE

_dec_b:
 dec b
 inc b
jnz _subtract

;b is zero here

 _b_zero_dec_a:
  dec a
  inc a
 jnz _b_zero_a_non_zero

;a and b are zero here

  ;; C is the result
  inc c
  jnz _result

 _b_zero_a_non_zero:
  dec a
  inc c
 jnz _b_zero_dec_a

  ;CANNOT FALL HERE


_subtract:
 dec a
 dec b
jnz _dec_a

 ;Result
_result:
 dec c