https://codeforces.com/contest/1582/problem/C
#include<iostream>
#include<algorithm>
#include<bits/stdc++.h>
using namespace std;

const int N = 1e3 + 10;
typedef long long ll;


int f[N][N];
 
 
 void solve() {
 	int n; 
 	string s;
 	cin >> n >> s;
 	int ans = n + 1;
	 for (int c = 0; c < 26; c ++) {
 		int l = 0, r = n - 1;
 		int cnt = 0;
		 while(l <= r) {
 			if (s[l] == s[r]) {
 				l ++, r --;
			 }
			 else if (s[l] == char(c + 'a')) {
			 	l ++;
			 	cnt ++;
			 }
			 else if (s[r] == char(c + 'a')) {
			 	r --;
			 	cnt ++;
			 }
			 else {
			 	cnt = n + 1;
			 	break;
			 }
		 }
		 ans = min(ans, cnt);
	 }
	 if (ans == n + 1) ans = - 1;
	cout << ans << endl;
 }
int main () {
	int t;
	cin >> t;
	while (t --) solve();
	return 0;
} 

因为最终只删除一个字母，而字母表只有26个，所以可以遍历所有删除的可能。然后取最小值