Given the root of a binary tree, return the sum of every tree node's tilt.
The tilt of a tree node is the absolute difference between the sum of all left subtree node values and all right subtree node values. If a node does not have a left child, then the sum of the left subtree node values is treated as 0. The rule is similar if the node does not have a right child.
[0, 10^4]-1000 <= Node.val <= 1000root = [1,2,3]1root = [4,2,9,3,5,null,7]15root = [21,7,14,1,1,2,2,3,3]9