[Leetcode] Binary tree-- 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:

Input: 
         1
       /   \
      2     3
Output: 1
Explanation: 
Tilt of node 2 : 0
Tilt of node 3 : 0
Tilt of node 1 : |2-3| = 1
Tilt of binary tree : 0 + 0 + 1 = 1


Solution:

1. naive solution,  recursively get every node's tilt  by treating every node as a root and traverse the left subtree and right subtree,

there is gonna be too much repetitions

  #1st niave recursive
        def findTiltSumHelper(root):
            if root is None:
                return 0
            return findTiltSumHelper(root.left) + root.val + findTiltSumHelper(root.right)
        
        if root is None:
            return 0
        return self.findTilt(root.left) + abs(findTiltSumHelper(root.left) - findTiltSumHelper(root.right)) + self.findTilt(root.right)

2. 

        def findTiltSumHelper(root):
            if root is None:
                return 0
            left = findTiltSumHelper(root.left) 
            right = findTiltSumHelper(root.right)
            tmp = abs(left - right)
            self.ansSum += tmp
            return left + root.val + right
        
        findTiltSumHelper(root)
        return  self.ansSum