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.

Note: A leaf is a node with no children.

Example:

**Input:** [1,2,3]
    1
   / \
  2   3
**Output:** 25
**Explanation:**
The root-to-leaf path `1->2` represents the number `12`.
The root-to-leaf path `1->3` represents the number `13`.
Therefore, sum = 12 + 13 = `25`.

Example 2:

**Input:** [4,9,0,5,1]
    4
   / \
  9   0
 / \
5   1
**Output:** 1026
**Explanation:**
The root-to-leaf path `4->9->5` represents the number 495.
The root-to-leaf path `4->9->1` represents the number 491.
The root-to-leaf path `4->0` represents the number 40.
Therefore, sum = 495 + 491 + 40 = `1026`.

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

class Solution:
    def sumNumbers(self, root: TreeNode) -> int:
        def getNumbers(root, previous, result):
            if not root:
                return
            
            current = previous*10 + root.val
            
            if not root.left and not root.right:
                result[0] += current
            
            getNumbers(root.left, current, result)
            getNumbers(root.right, current, result)
            
        ans = [0]
        getNumbers(root, 0, ans)
        return ans[0] 

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