Pinterest Machine Learning Engineer interview experience

I wasn’t allowed to rely on existing matrix or linear algebra libraries, so I had to think carefully about an internal data structure that stores only non-zero elements instead of allocating space for the full matrix.

After laying out the representation, I had to implement core operations—addition and multiplication—while making sure sparsity was handled correctly. The tricky part was maintaining performance while also staying accurate, even when the two matrices had different sparsity patterns.

It was the kind of problem that felt very hands-on and systems-minded rather than purely