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# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def zigzagLevelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if root == None:
            return []
        self.res = []
        self.DFS(root, 1, True)
        return self.res
    def DFS(self, node, level, left2Right):
        if node == None:
            return
        if len(self.res) < level:
            self.res.append([])
        if left2Right:
            self.res[level-1].append(node.val)
        else:
            self.res[level-1].insert(0, node.val)
        self.DFS(node.left, level+1, not left2Right)
        self.DFS(node.right, level+1, not left2Right)