This version now runs in 29s on my computer, 8s less than the previous version. And RAM usage went down to 60MB (-15GB).
% hyperfine -w 1 -r 5 './exe-exe > ../solution.txt'
Benchmark 1: ./exe-exe > ../solution.txt
Time (mean ± σ): 28.535 s ± 0.023 s [User: 25.609 s, System: 2.818 s]
Range (min … max): 28.505 s … 28.556 s 5 runs