Vertical Order Traversal of a Binary Tree — Python

# Time:  O(nlogn)
# Space: O(n)

import collections


# 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 verticalTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        def dfs(node, lookup, x, y):
            if not node:
                return
            lookup[x][y].append(node)
            dfs(node.left, lookup, x-1, y+1)
            dfs(node.right, lookup, x+1, y+1)
                
        lookup = collections.defaultdict(lambda: collections.defaultdict(list))
        dfs(root, lookup, 0, 0)

        result = []
        for x in sorted(lookup):
            report = []
            for y in sorted(lookup[x]):
                report.extend(sorted(node.val for node in lookup[x][y]))
            result.append(report)
        return result