Integer factorization is also reducible to Knapsack:

To factorize integer N invoke a Knapsack solver with knapsack size of log(N) and items of size logarithm of all prime numbers up to sqrt(N): [log 2, log 3, log 5, ...].

If N=pq (say p and q are prime) then log(N)=log(p)+log(q).

So from all possible items in the item set, only log(p) and log(q) will fill the knapsack as tight as possible leaving zero empty space in it.