LeetCode: 112. Path Sum

The problem is here: https://leetcode.com/problems/path-sum/ . Fancy that the number can be negative numbers, I made a stupid 'optimization'. I added a piece of code to check whether current node value is bigger than sum, assuming that every node value is positive. After droping this 'optimization', it works.

# 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 hasPathSum(self, root, sum):
        :type root: TreeNode
        :type sum: int
        :rtype: bool
        if root is None:
            return False

        if root.left is None \
            and root.right is None \
            and root.val == sum:
            return True

        return self.hasPathSum(root.left, sum-root.val) \
            or self.hasPathSum(root.right, sum-root.val)

I think maybe this is a DSP?