563. Binary Tree Tilt

Given a binary tree, return the tilt of the whole tree.

The tilt of a tree node is defined as the absolute difference between the sum of all left subtree node values and the sum of all right subtree node values. Null node has tilt 0.

The tilt of the whole tree is defined as the sum of all nodes' tilt.

Example:

可以先遍历一遍求sum，然后再遍历一遍求titlt

# Definition for a binary tree node.
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution:
    def findTilt(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root: return 0
        return self.findTilt(root.left)+self.findTilt(root.right)+\
                abs(self.sum(root.left)-self.sum(root.right))
    
    def sum(self, root):
        if not root: return 0
        return self.sum(root.left)+root.val+self.sum(root.right)