Binary Tree Maximum Path Sum - Leetcode 124 - Python

A path in a binary tree is a sequence of nodes where each pair of adjacent nodes in the sequence has an edge connecting them. A node can only appear in the sequence at most once. Note that the path does not need to pass through the root.

The path sum of a path is the sum of the node's values in the path.

Given the root of a binary tree, return the maximum path sum of any non-empty path. 

Input: root = [-10,9,20,null,null,15,7]
Output: 42
Explanation: The optimal path is 15 -> 20 -> 7 with a path sum of 15 + 20 + 7 = 42.

class Solution:
    def maxPathSum(self, root: Optional[TreeNode]) -> int:
        ans = [root.val]
        
        #max w/o split
        def dfs(root):
            if not root :
                return 0
            
            ml = dfs(root.left)
            ml = max(ml, 0)
            
            mr = dfs(root.right)
            mr = max(mr, 0)
            
            #max with split
            ans[0] = max(ans[0], root.val + ml + mr)
            
            return root.val + max(ml, mr)
        
        dfs(root)
        return ans[0]

Explaination :