二叉树之字型层次遍历

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root: return []
        result = []
        cur_level_nodes = [root]
        flag = True
        while cur_level_nodes:
            cur_level_vals = []
            next_level_nodes = []
            for node in cur_level_nodes:
                cur_level_vals.append(node.val)
                if node.left:
                    next_level_nodes.append(node.left)
                if node.right:
                    next_level_nodes.append(node.right)
            
            cur_level_nodes = next_level_nodes
            if flag == True:
                result.append(cur_level_vals)
                flag = False
            else:
                result.append(cur_level_vals[::-1])
                flag = True

        
        return result