CF1271B.Blocks

There are $n$ blocks arranged in a row and numbered from left to right, starting from one. Each block is either black or white.

You may perform the following operation zero or more times: choose two adjacent blocks and invert their colors (white block becomes black, and vice versa).

You want to find a sequence of operations, such that they make all the blocks having the same color. You don't have to minimize the number of operations, but it should not exceed $3 \cdot n$. If it is impossible to find such a sequence of operations, you need to report it.

有 $n$ 个方块从左到右排成一行，并从左至右依次编号为 $1$ 到 $n$。每个方块为黑色或白色。

你可以执行以下操作零次或多次：选择两个相邻的方块，并将它们的颜色翻转（白色变为黑色，黑色变为白色）。

你需要找到一个操作序列，使得所有方块最终颜色相同。你无需最小化操作次数，但操作总数不得超过 $3 \cdot n$。若不存在满足条件的操作序列，则需报告该情况。

The first line contains one integer $n$ ($2 \le n \le 200$) — the number of blocks.

The second line contains one string $s$ consisting of $n$ characters, each character is either "W" or "B". If the $i$-th character is "W", then the $i$-th block is white. If the $i$-th character is "B", then the $i$-th block is black.

第一行包含一个整数 $n$（$2 \le n \le 200$）—— 表示方块的数量。

第二行包含一个由 $n$ 个字符组成的字符串 $s$，每个字符为 "W" 或 "B"。如果第 $i$ 个字符是 "W"，则第 $i$ 个方块为白色；如果第 $i$ 个字符是 "B"，则第 $i$ 个方块为黑色。

If it is impossible to make all the blocks having the same color, print $-1$.

Otherwise, print an integer $k$ ($0 \le k \le 3 \cdot n$) — the number of operations. Then print $k$ integers $p_1, p_2, \dots, p_k$ $(1 \le p_j \le n - 1)$, where $p_j$ is the position of the left block in the pair of blocks that should be affected by the $j$-th operation.

If there are multiple answers, print any of them.

如果无法使所有方块颜色相同，则输出 $-1$。

否则，输出一个整数 $k$（$0 \le k \le 3 \cdot n$）—— 表示操作次数。然后输出 $k$ 个整数 $p_1, p_2, \dots, p_k$（$1 \le p_j \le n - 1$），其中 $p_j$ 表示第 $j$ 次操作所作用的两个方块中左侧方块的位置。

若存在多个答案，输出任意一个即可。

In the first example, it is possible to make all blocks black in $3$ operations. Start with changing blocks $6$ and $7$, so the sequence is "BWWWWBBB". Then change blocks $2$ and $3$, so the sequence is "BBBWWBB". And finally, change blocks $4$ and $5$, so all blocks are black.

It is impossible to make all colors equal in the second example.

All blocks are already white in the third example.

In the fourth example it is possible to make all blocks white in two operations: first operation is to change blocks $2$ and $3$ (so the sequence is "BBW"), and then change blocks $1$ and $2$ (so all blocks are white).

在第一个例子中，可以通过 $3$ 次操作使所有方块变为黑色。首先翻转方块 $6$ 和 $7$，序列变为 “BWWWWBBB”；接着翻转方块 $2$ 和 $3$，序列变为 “BBBWWBB”；最后翻转方块 $4$ 和 $5$，所有方块均变为黑色。

在第二个例子中，无法使所有方块颜色相同。

在第三个例子中，所有方块初始即为白色。

在第四个例子中，可以通过两次操作使所有方块变为白色：第一次操作翻转方块 $2$ 和 $3$（序列变为 “BBW”），第二次操作翻转方块 $1$ 和 $2$（所有方块均变为白色）。