Quite a fun challenge for those looking to understand Programming Language Concepts, Parsing, and Tree building!
Nice work.

I'm surprised not many people forwent importing re,

My solution implements a recursive algorithm that solves from the inside out.

I would like to know the benefits in using regex vs recursive decent for parsing problems like this one

Well if you got a SIGSEGV (signal: segmentation violation), then it's likely that your program tried to access a spot in the vector numbers that was out of bounds. This might be because:

a) you used ++i in the for loop control instead of i++

or

b) you didn't include the -1 in the for loop control (needed to stay in bounds when grabbing [i+1] inside the loop)

Hope this was helpful!

still an issue after 6 months...