Insufficient Nodes in Root to Leaf Paths — Python

# Time:  O(n)
# Space: O(h)

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


class Solution(object):
    def sufficientSubset(self, root, limit):
        """
        :type root: TreeNode
        :type limit: int
        :rtype: TreeNode
        """
        if not root:
            return None
        if not root.left and not root.right:
            return None if root.val < limit else root
        root.left = self.sufficientSubset(root.left, limit-root.val)
        root.right = self.sufficientSubset(root.right, limit-root.val)
        if not root.left and not root.right:
            return None
        return root