Range Sum BST

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def rangeSumBST(self, root, low, high):
        """
        :type root: TreeNode
        :type low: int
        :type high: int
        :rtype: int
        """
        self.out = 0
        def dfs(node):
           if not node:
               return
           if low<=node.val<=high:
               self.out += node.val
           if node.val>low:
               dfs(node.left)
           if node.val<high:
               dfs(node.right)
        dfs(root)
        return self.out