Breadth-first search! Of an arbitrary (not necessarily binary) tree. Glad to have this one. At least in the python implementation it’s weird that they model null as [], it should be None, but whatever.

def tree_to_list(tree_root):
    res = []
    worklist = [tree_root]
    while worklist:
        n = worklist[0]
        worklist = worklist[1:]
        if n == []:
            continue
        res.append(n.data)
        for c in n.child_nodes:
            worklist.append(c)
    return res