Problem URL: Binary tree tilt

We will use DFS to find the tilt of each subtree and then add them together.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

class Solution:
    def findTilt(self, root: Optional[TreeNode]) -> int:
        self.tiltSum = 0

        def dfs(root: Optional[TreeNode]) -> int:
            if not root:
                return 0

            left = dfs(root.left)
            right = dfs(root.right)
            self.tiltSum += abs(left - right)

            return root.val + left + right

        dfs(root)
        return self.tiltSum

Time complexity: O(n)
Space complexity: O(n)