How many numbers below $100$ can be expressed as a difference of two perfect squares in only one way? 
How many numbers below $100$ can be expressed as a difference of two
  perfect squares in only one way?

Please explain your approach.
ADDED: I can determine whether a number could be represented as difference of two squares,say $N = a^2-b^2$ ,where $a,b,N \in \mathbb{N}$,which gives $N = (a+b)(a-b)$ hence,$(a+b)$ and $(a-b)$ must be both odd or both even,observing the prime factorization of $N$ we can determine whether it is possible to represent $N$ in that form or not,but I am not sure if I could use this for this problem.

HINT:
The differences between perfect squares conform to a strict pattern. 
$1^2 = 1$  
$2^2 = 1^2 + 3$  
$3^2 = 2^2 + 5 = 1^2 + 8$ 
$4^2 = 3^2 + 7 = 2^2 + 12 = 1^2 + 15$  
$5^2 = 4^2 + 9 = 3^2 + 16 = 2^2 + 21 = 1^2 + 24$
One could write a program to test the answers if you wanted, as above a certain number $m$ (I'll let you find out what that is - though it isn't so bad) the possible differences of it's square with other squares are simply too big. It's okay, too, since this number is not very big.
Second HINT:
$x^2 - y^2 = (x+y)(x-y)$
