Weave two lists

a and b are the heads of two linked lists. Either may be None (an empty list). a is as long as b, or one node longer.

Write weave(a, b) that rewires the existing nodes so they alternate, starting with a: a1, b1, a2, b2, … Return the head (still the first node of a). No new nodes.

1 -> 2 -> 3 and 7 -> 8 → 1 -> 7 -> 2 -> 8 -> 3