124. 二叉树中的最大路径和

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

class Solution:

    def maxPathSum(self, root: TreeNode) -> int:
        def dfs(root):# dfs表示从当前节点往下走的某条路的最大值
            nonlocal res
            if not root: return 0
            a=dfs(root.left)
            b= dfs(root.right)
            res= max(res,root.val+a+b)
            return max(0,root.val+max(a,b))
        res=float("-inf")     
        dfs(root)
        return res