Copy List with Random Pointer

Each node has a `next` and a `random` pointer that may target any node or `None`. Write `copy_random_list(head)` returning a deep copy: entirely new nodes, with the same structure. The difficulty is that a random pointer may target a node you have not created yet. Two passes with a map from original node to copy solves it. The O(1)-space version interleaves each copy directly after its original, so `copy.random = original.random.next` — worth knowing, and worth saying you know.