987. Vertical Order Traversal of a Binary Tree - LeetCode Solution

# 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 verticalTraversal(self, root: TreeNode) -> List[List[int]]:
        
        def dfs(root, xpos, ypos):
            if not root:
                return 
            
            d[xpos].append((ypos, root.val))
            dfs(root.left,xpos-1,ypos+1)
            dfs(root.right,xpos+1,ypos+1)
        
        d = defaultdict(list)
        
        dfs(root,0,0)            
        return [ [z for y,z in sorted(ve)] for kay, ve in sorted(d.items())]