The C++ submission you linked does not use any hashmap. It just uses a couple of vectors.

Instead of hashmaps you should use arrays. Note that $$$a_i \leq n$$$, now we can use the fact that xor of a set of elements $$$\leq n$$$ is $$$\leq 2^{\lfloor \log n \rfloor + 1} - 1$$$.

So if you use an array of size $$$2^{\lfloor \log n \rfloor + 1}$$$ you would be safe. (in the official solution they have used $$$2n$$$ which is $$$> 2^{\lfloor \log n \rfloor + 1} - 1$$$).

Hashmaps are usually quite slow and take a lot of constant time, in fact some people who tried to use unordered_map of c++ also got TLE (you can see the editorial comment section).