二叉树与递归

def traverse(node):
  if not node: return
  # 前序位置
  traverse(node.left)
  # 中序位置
  traverse(node.right)
  # 后序位置

def solve(node):
  if not node: return base
  left = solve(node.left)
  right = solve(node.right)
  update_answer(left, right)
  return combine(left, right)

queue = [root]
while queue:
  size = len(queue)
  for _ in range(size):
    node = pop_front()
    push_children(node)