129 Sum Root to Leaf Numbers

class Solution:
    def __init__(self):
        self.ans = 0

    def sumNumbers(self, root):
        self.help(root, "")
        return self.ans

    def help(self, root, s):
        if root == None:
            return
        v = str(root.val)
        if root.left == None and root.right == None:
            self.ans += int(s+v)
        elif root.left == None and root.right != None:
            self.help(root.right, s+v)
        elif root.left != None and root.right == None:
            self.help(root.left, s+v)
        else:
            self.help(root.left, s+v)
            self.help(root.right, s+v)