Finding suitable NP-complete problem to reduce to my problem I've been given a set $S$ of natural numbers (non-negative integers) $s_1$, $s_2$,...,$s_n$ where $|S|=n$. My problem is to figure out if there is way to get a total sum of 0 when using all the numbers of $S$ but we get to "choose the sign" for each number: that is, for $s_i$ we get to choose if we're going to add or subtract $s_i$ from the total sum. 

I've tried visualizing it using a directed cyclic graph with $n$ vertices where the only edges from $v_1$ to $v_2$ are $+/- s_1$ and the task is to traverse the whole graph, ultimately getting back to our starting node $v_1$ using a path with total weight 0. 

What would be a suitable NP-complete problem to reduce into this one?
This problem is essentially just a rephrasing of the classic NP-complete problem PARTITION. If you can partition a set $S$ of natural numbers into two partitions $S_1$ and $S_2$ with equal sum, then $\sum_{s \in S_1} s + \sum_{s \in S_2} -s = 0$ and vice versa.