101 && 202 can be decoded with 2 ways but acc. to problem its 1


#1

101 - 10, 1 ja
101 - 1 ,01 aa


#2

01 should not be considered i.e. a is represented by 1 not 01 or 001.
eg: 403 has zero decoding


#3

found a really nice explanation of this question check it out.