sum of left leaves

def (root):
    res = list()
    if not root:
        return res
    if root.left is not None and root.left.left is None and root.left.right is None:
        res.append(root.left.val)
    res += traversal(root.left)
    res += traversal(root.right)
    return res

def SumOfLeftLeaves(root):
    leaves = traversal(root)
    return sum(leaves)