Path Sum III

You are given a binary tree in which each node contains an integer value.

Find the number of paths that sum to a given value.

The path does not need to start or end at the root or a leaf, but it must go downwards (traveling only from parent nodes to child nodes).

The tree has no more than 1,000 nodes and the values are in the range -1,000,000 to 1,000,000.
root = [10,5,-3,3,2,null,11,3,-2,null,1], sum = 8
10
/ \
5 -3
/ \ \
3 2 11
/ \ \
3 -2 1

Return 3. The paths that sum to 8 are:

1. 5 -> 3
2. 5 -> 2 -> 1
3. -3 -> 11

class Solution(object):
    def pathSum(self, root, sum):
        """
        :type root: TreeNode
        :type sum: int
        :rtype: int
        """
        self.pathSum=0;
        self.dfs(root,sum)
        return self.pathSum
    def dfs(self,root,target):
        if(root==None):
            return 
        self.test(root,target)
        self.dfs(root.left,target)
        self.dfs(root.right,target)
    def test(self,root,target):
        if(root==None):
            return
        if(root.val==target):
            self.pathSum+=1
        self.test(root.left,target-root.val)        
        self.test(root.right,target-root.val)