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Leetcode Hot100 - 124. 二叉树中的最大路径和

# 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.ans = -0x3f3f3f3f

        def foo(root):
            if root is None:
                return 0
            max_left = foo(root.left)
            max_right = foo(root.right)

            node_sum = root.val
            path_sum = root.val
            if max_left > 0:
                node_sum += max_left
            if max_right > 0:
                node_sum += max_right

            self.ans = max(self.ans, node_sum)
            if max_left < 0 and max_right < 0:
                return path_sum
            return path_sum + max(max_left, max_right)
        
        foo(root)
        return self.ans