Maximum path sum from any node

'''
for each node there can be four ways that the max path goes through the node:
1)Node only
2)Max path through left child+node
3)max path  through right child + node
4)max path through left child+node+max path through right child
Note: updating max without including the root with only n1 and n2 is not the current node responsibility,it would have
been already done on the descendent node.
'''
'''
class Node:
    def __init__(self,val):
        self.data = val
        self.left = None
        self.right = None
'''
def fun(root, ans):
    if not root:
        return 0
    l1,l2 = 0,0
    if root.left:
        l1 = fun(root.left, ans)
    if root.right:
        l2 = fun(root.right, ans)

    # best path that can be extended to parent
    cur_node = max(root.data,root.data + max(l1, l2))

    # path passing through current node
    cross_path = l1 + l2 + root.data
    # update global answer
    ans[0] = max(ans[0],cur_node,cross_path)
    return cur_node

class Solution:
    def findMaxSum(self, root):
        if not root:
            return 0
        ans = [float('-inf')]
        fun(root, ans)
        return ans[0]