Spot the correct code.

Three implementations. One of them is right.

The wrong two are the correct code with between two and five lines changed — a pop_back at the bottom of a recursion that never runs, a bound that moves by one, a reference that will not bind. Difficulty is the closest decoy: two changed lines is Hard, because an option wrong in only two places is the one that survives a careless read.

The problem is named here, unlike the other mode — the statement is on the page anyway, so there is nothing left for a name to give away.