def last_kth(link, k):
    if not link or k <= 0:
        return None
    move = link
    while move and k-1 >= 0:
        move = move.next
        k -= 1
    while move:
        move = move.next
        link = link.next
    if k == 0:
        return link.val
    return None

def reverse_link(head):
    if not head or not head.next:
        return head
    then = head.next
    head.next = None
    last = then.next
    while then:
        then.next = head
        head = then
        then = last
        if then:
            last = then.next
    return head

def merge_link(head1, head2):
    if not head1:
        return head2
    if not head2:
        return head1
    if head1.val <= head2.val:
        ret = head1
        ret.next = merge_link(head1.next, head2)
    else:
        ret = head2
        ret.next = merge_link(head1, head2.next)
    return ret

def sub_tree(tree1, tree2):
    if tree1 and tree2:
        if tree1.val == tree2.val:
            return sub_tree(tree1.left, tree2.left) and sub_tree(tree1.right, tree2.right)
        else:
            return sub_tree(tree1.left, tree2) or sub_tree(tree1.right, tree2)
    if not tree1 and tree2:
        return False
    return True