Trees: BFS & DFS

def max_depth(r):
    if not r: return 0
    return 1+max(max_depth(r.left), max_depth(r.right))

from collections import deque
def levels(r):
    q, out = deque([r]), []
    while q:
        out.append([n.v for n in q])
        for _ in range(len(q)):
            n=q.popleft()
            if n.left: q.append(n.left)
            if n.right: q.append(n.right)
    return out