LeetCode 494 Target Sum - Converting Signs to Subsets

Let $P$ = sum of numbers with + sign, $N$ = sum of numbers with - sign. You know: $P - N = target$ and $P + N = total$. Add these equations: $2P = target + total$, so $P = (target + total) / 2$. If this isn't an integer, return 0.

Now the problem becomes: count subsets that sum to $P$. That's subset sum with counting instead of boolean. The transformation is complete. You're selecting which numbers get the + sign.