124. Binary Tree Maximum Path Sum

Problem

# 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: Optional[TreeNode]) -> int:
        self.res = -sys.maxsize
        self.fromRoot(root)
        return self.res

    def fromRoot(self, root) -> int:
        if not root:
            return 0
        left = self.fromRoot(root.left)
        right = self.fromRoot(root.right)
        pathSum = root.val
        if left > 0:
            pathSum += left
        if right > 0:
            pathSum += right
        self.res = max(self.res, pathSum)

        more = max(0, left, right)
        return root.val + more