def verticalTraversal(root):
    # Level-order traversal, keeping track of xpos
    q = deque([(root, 0)])
    # Record columns
    # Key is xpos of col
    # Val is list of vals in that col, sorted from top to bottom
    cols = defaultdict(list)
    while q:
        n = len(q)
        for i in range(n):
            node, x = q.popleft()
            cols[x].append(node.val)
            # Append children to queue, with their xpos
            if node.left:
                q.append((node.left, x - 1))
            if node.right:
                q.append((node.right, x + 1))
    # Format answer
    ans = [cols[x] for x in sorted(cols.keys())]
    return ans