129. Sum Root to Leaf Numbers.py

'''
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, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

answer = 0
class Solution:
    def sumNumbers(self, root: TreeNode) -> int:
        global answer
        answer = 0
        DFS(root, 0)
        return answer
        
        
def DFS(root, cur):
    global answer
    if root==None:
        return
    
    if root.left==None and root.right==None:
        cur = (cur*10)+root.val
        answer+=cur
        return
        
    DFS(root.left, (cur*10)+root.val)
    DFS(root.right, (cur*10)+root.val)