[Leetcode][python]Sum Root to Leaf Numbers

class Solution(object):
    def sumNumbers(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        return self._sumNumbers(root,0)

    def _sumNumbers(self, root, preSum):
        if not root:
            return 0
        preSum = preSum * 10 + root.val
        if root.left == None and root.right == None:
            return preSum
        return self._sumNumbers(root.left, preSum) + self._sumNumbers(root.right, preSum)