## 题目

Given a binary tree, return the vertical order traversal of its nodes values.

For each node at position (X, Y), its left and right children respectively will be at positions (X-1, Y-1) and (X+1, Y-1).

Running a vertical line from X = -infinity to X = +infinity, whenever the vertical line touches some nodes, we report the values of the nodes in order from top to bottom (decreasing Y coordinates).

If two nodes have the same position, then the value of the node that is reported first is the value that is smaller.

Return an list of non-empty reports in order of X coordinate. Every report will have a list of values of nodes.

Example 1:

Example 2:

Note:

1. The tree will have between 1 and 1000 nodes.
2. Each node’s value will be between 0 and 1000.

## 思路

DFS.
I have a dictionary with the keys: x and the value: a tuple of (y, root.val). I then sort the dictionary based on keys and sort it again based on y.