Binary Tree Tilt

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def findTilt(self, root, res =0):
        """
        :type root: TreeNode
        :rtype: int
        """
        self.res = 0

        def helper(root):
            if not root:
                return 0

            #postorder
            ls = helper(root.left)
            rs = helper(root.right)
            self.res += abs(ls - rs)

            return ls + rs + root.val

        helper(root)
        return self.res