CF2162B.Beautiful String

You are given a binary$^{\text{∗}}$ string $s$ of length $n$.

Your task is to find any subsequence$^{\text{†}}$ $p$ of $s$ such that:

• The subsequence $p$ is non-decreasing. That is, each character in $p$ is not greater than the next one.
• Let $x$ denote the string obtained by removing all characters of $p$ from $s$, while preserving the order of the remaining characters. Then $x$ must be a palindrome$^{\text{‡}}$.

You only need to output any valid subsequence $p$ that satisfies both conditions. If no such subsequence exists, output $-1$.

Note that an empty string is both non-decreasing and a palindrome.

$^{\text{∗}}$A binary string is a string consisting of characters '0' and '1'.

$^{\text{†}}$A subsequence of a string $s = s_1s_2\ldots s_n$ is a sequence $p = s_{i_1}s_{i_2}\ldots s_{i_k}$ such that $1 \leq i_1 \lt i_2 \lt \ldots \lt i_k \leq n$. The characters are selected in order, but not necessarily contiguously. Note that an empty string is a subsequence of any string.

$^{\text{‡}}$A string $t = t_1t_2\ldots t_m$ is a palindrome if $t_i = t_{m - i + 1}$ for all $1 \leq i \leq m$. In other words, the string reads the same forward and backward.

给你一个长度为 $n$ 的二进制$^{\text{∗}}$字符串 $s$。

你的任务是找出 $s$ 的任意一个子序列$^{\text{†}}$ $p$，使得：

• 子序列 $p$ 是非递减的，即 $p$ 中每个字符都不大于其后一个字符；
• 设 $x$ 为从 $s$ 中删除 $p$ 的所有字符（保持剩余字符的原有顺序）后所得的字符串，则 $x$ 必须是一个回文串$^{\text{‡}}$。

你只需输出任意一个满足上述两个条件的子序列 $p$。若不存在这样的子序列，则输出 $-1$。

注意：空字符串既是非递减的，也是回文串。

$^{\text{∗}}$二进制字符串是指仅由字符 '0' 和 '1' 组成的字符串。

$^{\text{†}}$字符串 $s = s_1s_2\ldots s_n$ 的一个子序列是指形如 $p = s_{i_1}s_{i_2}\ldots s_{i_k}$ 的序列，其中 $1 \leq i_1 \lt i_2 \lt \ldots \lt i_k \leq n$。字符按原顺序选取，但不必连续。注意：空字符串是任意字符串的子序列。

$^{\text{‡}}$字符串 $t = t_1t_2\ldots t_m$ 是回文串，当且仅当对所有 $1 \leq i \leq m$，均有 $t_i = t_{m - i + 1}$。换言之，该字符串正读与反读完全相同。

The first line contains a single integer $t$ ($1 \le t \le 3000$) — the number of test cases.

The first line of each test case contains a single integer $n$ ($1 \le n \le 10$) — the length of the string.

The second line contains a binary string $s$ of length $n$.

第一行包含一个整数 $t$（$1 \le t \le 3000$）—— 测试用例的数量。

每个测试用例的第一行包含一个整数 $n$（$1 \le n \le 10$）—— 字符串的长度。

每个测试用例的第二行包含一个长度为 $n$ 的二进制字符串 $s$。

If a solution exists:

• On the first line, print a single integer $k$ ($0 \le k \le n$) — the length of the subsequence $p$.
• On the second line, print $k$ distinct integers $i_1, i_2, \dots, i_k$ ($1 \le i_1 \lt i_2 \lt \dots \lt i_k \le n$) — the indices of the characters in $s$ that form $p$ (in order as they appear in $s$).

Otherwise, print a single line containing $-1$.

如果存在解：

• 在第一行，输出一个整数 $k$（$0 \le k \le n$）——子序列 $p$ 的长度。
• 在第二行，输出 $k$ 个互不相同的整数 $i_1, i_2, \dots, i_k$（$1 \le i_1 \lt i_2 \lt \dots \lt i_k \le n$）——构成 $p$ 的字符串 $s$ 中字符的下标（按其在 $s$ 中出现的顺序）。

否则，输出一行，包含 $-1$。

In the first test case, we remove an empty string, resulting in $x = \texttt{010}$, which is a palindrome.

In the second test case, we remove $p = \texttt{01}$ (indices $2$, $3$), resulting in $x = \texttt{0}$, which is a palindrome.

In the third test case, we remove $p = \texttt{00111}$ (indices $1$ to $5$), resulting in an empty string, which is trivially a palindrome.

In the fourth test case, we remove $p = \texttt{01}$ (indices $3$, $4$), resulting in $x = \texttt{110011}$, which is a palindrome.

In the fifth test case, we remove $p = \texttt{01}$ (indices $5$, $6$), resulting in $x = \texttt{1001}$, which is a palindrome.

在第一个测试用例中，我们移除一个空字符串，得到 $x = \texttt{010}$，它是一个回文串。

在第二个测试用例中，我们移除 $p = \texttt{01}$（下标为 $2$、$3$），得到 $x = \texttt{0}$，它是一个回文串。

在第三个测试用例中，我们移除 $p = \texttt{00111}$（下标为 $1$ 至 $5$），得到一个空字符串，它显然也是一个回文串。

在第四个测试用例中，我们移除 $p = \texttt{01}$（下标为 $3$、$4$），得到 $x = \texttt{110011}$，它是一个回文串。

在第五个测试用例中，我们移除 $p = \texttt{01}$（下标为 $5$、$6$），得到 $x = \texttt{1001}$，它是一个回文串。