二叉树锯齿形层序遍历

def solve(____):
    from collections import deque
    if not root:
        return []
    result, q, left_to_right = [], deque(____), True
    while q:
        level = []
        for _ in range(len(____)):
            node = q.popleft()
            level.append(____)
            if node.left:
                q.append(____)
            if node.right:
                q.append(____)
        if not left_to_right:
            level.reverse()
        result.append(____)
        left_to_right = not left_to_right
    return result