LeetCode 124: Binary Tree Maximum Path Sum 2

I'm posting my code for a LeetCode problem. If you'd like to review, please do so. Thank you for your time!

Problem

Given a non-empty binary tree, find the maximum path sum.

For this problem, a path is defined as any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The path must contain at least one node and does not need to go through the root.

Example 1:

Input: [1,2,3]

1
/ \
2   3

Output: 6


Example 2:

Input: [-10,9,20,null,null,15,7]

-10
/ \
9  20
/  \
15   7

Output: 42


Inputs

[1,2,3]
[-10,9,20,null,null,15,7]
[-10,9,20,null,null,15,7,9,20,null,null,15,7]
[-10,9,20,null,null,15,7,9,20,null,null,15,720,null,null,15,7,9,20,null,null,15,7]
[-10,9,20,null,null,15,7,9,20,null,null,15,720,null,null,15,7,9,20,null,null,15,7999999,20,null,null,15,7,9,20,null,null,15,720,null,null,15,7,9,20,null,null,15,7]


Outputs

6
42
66
791
8001552


Code

# 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 maxPathSum(self, root: TreeNode) -> int:
def depth_first_search(node: TreeNode) -> int:
if not node:
return 0
left_sum = depth_first_search(node.left)
right_sum = depth_first_search(node.right)
if not left_sum or left_sum < 0:
left_sum = 0
if not right_sum or right_sum < 0:
right_sum = 0
self.sum = max(self.sum, left_sum + right_sum + node.val)
return max(left_sum, right_sum) + node.val

self.sum = float('-inf')
depth_first_search(root)
return self.sum



Unnecessary checks in depth_first_search()
The function depth_first_search() always returns an integer value, never None. So the check for a partial sum being None or < 0 can be rewritten using max():
left_sum = max(0, depth_first_search(node.left))