Considering the answer is 1 line, I doubt I would have hired him either... def inverse(t): return Node(inverse(t.r), t.v, inverse(t.l)) if t else t or if

Considering the answer is 1 line, I doubt I would have hired him either... def inverse(t): return Node(inverse(t.r), t.v, inverse(t.l)) if t else t or if you must do it in place, 3 lines... def invert(t): if t: (t.l, t.r) = (invert(t.r), t.v, invert(t.l)) return t