987.md

987. Vertical Order Traversal of a Binary Tree

Solution 1 (O(n*log(n)))

# BFS

# 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]]
        """
        cols = collections.defaultdict(list)
        q = [(root, 0)]
        while q:
            cur_cols = collections.defaultdict(list)
            for node, col in q:
                cur_cols[col].append(node.val)
            for c in cur_cols.keys():
                cols[c].extend(sorted(cur_cols[c]))
            new_q = []
            for node, col in q:
                if node.left is not None:
                    new_q.append((node.left, col - 1))
                if node.right is not None:
                    new_q.append((node.right, col + 1))
            q = new_q
        return [cols[c] for c in sorted(cols.keys())]