A15931. Making Anti-Palindromes
编程题
普及/提高-
知识点
题目描述
You are given a string $s$ , consisting of lowercase English letters. In one operation, you are allowed to swap any two characters of the string $s$ .
A string $s$ of length $n$ is called an anti-palindrome, if $s[i] \ne s[n - i + 1]$ for every $i$ ( $1 \le i \le n$ ). For example, the strings "codeforces", "string" are anti-palindromes, but the strings "abacaba", "abc", "test" are not.
Determine the minimum number of operations required to make the string $s$ an anti-palindrome, or output $-1$ , if this is not possible.
A string $s$ of length $n$ is called an anti-palindrome, if $s[i] \ne s[n - i + 1]$ for every $i$ ( $1 \le i \le n$ ). For example, the strings "codeforces", "string" are anti-palindromes, but the strings "abacaba", "abc", "test" are not.
Determine the minimum number of operations required to make the string $s$ an anti-palindrome, or output $-1$ , if this is not possible.
输入格式
The first line contains a single integer $t$ ( $1 \le t \le 10^4$ ) — the number of test cases. The description of the test cases follows.
Each test case consists of two lines. The first line contains a single integer $n$ ( $1 \le n \le 2 \cdot 10^5$ ) — the length of the string $s$ .
The second line contains the string $s$ , consisting of $n$ lowercase English letters.
The sum of $n$ over all test cases does not exceed $2 \cdot 10^5$ .
Each test case consists of two lines. The first line contains a single integer $n$ ( $1 \le n \le 2 \cdot 10^5$ ) — the length of the string $s$ .
The second line contains the string $s$ , consisting of $n$ lowercase English letters.
The sum of $n$ over all test cases does not exceed $2 \cdot 10^5$ .
输出格式
For each test case, output a single integer — the minimum number of operations required to make the string $s$ an anti-palindrome, or $-1$ if this is not possible.
输入输出样例
输入 #1
10 10 codeforces 3 abc 10 taarrrataa 10 dcbdbdcccc 4 wwww 12 cabbaccabaac 10 aadaaaaddc 14 aacdaaaacadcdc 6 abccba 12 dcbcaebacccd
输出 #1
0 -1 1 1 -1 3 -1 2 2 2
说明/提示
In the first test case, the string "codeforces" is already an anti-palindrome, so the answer is $0$ .
In the second test case, it can be shown that the string "abc" cannot be transformed into an anti-palindrome by performing the allowed operations, so the answer is $-1$ .
In the third test case, it is enough to swap the second and the fifth characters of the string "taarrrataa", and the new string "trararataa" will be an anti-palindrome, so the answer is $1$ .
In the second test case, it can be shown that the string "abc" cannot be transformed into an anti-palindrome by performing the allowed operations, so the answer is $-1$ .
In the third test case, it is enough to swap the second and the fifth characters of the string "taarrrataa", and the new string "trararataa" will be an anti-palindrome, so the answer is $1$ .