Depth-first search can be written recursively with no visible stack, or iteratively with an explicit one. What is the recursive version actually using?

A. The call stack, borrowed to remember where to resume

Each frame holds the node you must come back to, which is exactly what the explicit stack holds, same LIFO discipline, different owner. Recursion is more readable and bounded by stack depth; the explicit version can be given a heap-allocated stack, which is how graph traversal survives a million-node input.