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All leaked interview problems are collected from Internet.

Given a binary tree, return the *vertical order* traversal of its nodes' values. (ie, from top to bottom, column by column).

If two nodes are in the same row and column, the order should be from **left to right**.

**Examples:**

- Given binary tree
`[3,9,20,null,null,15,7]`

,

3 /\ / \ 9 20 /\ / \ 15 7

return its vertical order traversal as:

[ [9], [3,15], [20], [7] ]

- Given binary tree
`[3,9,8,4,0,1,7]`

,

3 /\ / \ 9 8 /\ /\ / \/ \ 4 01 7

return its vertical order traversal as:

[ [4], [9], [3,0,1], [8], [7] ]

- Given binary tree
`[3,9,8,4,0,1,7,null,null,null,2,5]`

(0's right child is 2 and 1's left child is 5),

3 /\ / \ 9 8 /\ /\ / \/ \ 4 01 7 /\ / \ 5 2

return its vertical order traversal as:

[ [4], [9,5], [3,0,1], [8,2], [7] ]

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