Sum of Left Leaves — Python

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

class Solution(object):
    def sumOfLeftLeaves(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        def sumOfLeftLeavesHelper(root, is_left):
            if not root:
                return 0
            if not root.left and not root.right:
                return root.val if is_left else 0
            return sumOfLeftLeavesHelper(root.left, True) + \
                   sumOfLeftLeavesHelper(root.right, False)

        return sumOfLeftLeavesHelper(root, False)