Solving "Two Sum" in C with a tiny hash table

I came across a question: How does one efficiently solve Two Sum in C? There’s a naive quadratic time solution, but also an amortized linear time solution using a hash table. Without a built-in or standard library hash table, the latter sounds onerous. However, a mask-step-index table, a hash table construction suitable for many problems, requires only a few lines of code. This approach is useful even when a standard hash table is available, because by exploiting the known problem constraints, i...