Leetcode 129 Sum Root to Leaf Numbers

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path 1->2->3 which represents the number 123.

Find the total sum of all root-to-leaf numbers.

For example,

    1
   / \
  2   3

 

The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.

Return the sum = 12 + 13 = 25.

class Solution(object):
    def sumNumbers(self, root):
        return self.sn(0,root)
    
    def sn(self,num,root):
        if not root:
            return 0
        num = num*10 + root.val
        if not root.left and not root.right:
            return num
        return self.sn(num,root.left) + self.sn(num,root.right)