HDU 5202 Rikka with string

Problem Description:

As we know, Rikka is poor at math. Yuta is worrying about this situation, so he gives Rikka some math tasks to practice. There is one of them:

One day, Yuta got a string which contains n letters but Rikka lost it in accident. Now they want to recover the string. Yuta remembers that the string only contains lowercase letters and it is not a palindrome string. Unfortunately he cannot remember
some letters. Can you help him recover the string?

It is too difficult for Rikka. Can you help her?
Input:

This problem has multi test cases (no more than
20).
For each test case, The first line contains a number
n(1≤n≤1000).
The next line contains an n-length string which only contains lowercase letters and ‘?’ – the place which Yuta is not sure.

Output:

For each test cases print a n-length string – the string you come up with. In the case where more than one string exists, print the lexicographically first one. In the case where no such string exists, output “QwQ”.

Sample Input:

5

a?bb?

3

aaa

Sample Output:

aabba

QwQ

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <algorithm>
#define LL long long
#define FOR(i, x, y) for(int i=x;i<=y;i++)
using namespace std;
const int MAXN = 1000 + 10;
char s[MAXN];
int n;
int ok;
bool check()
{
for(int i=0;i<=n/2;i++)
{
if(s[i] != s[n-1-i])
return false;
}
return true;
}
void dfs(int x)
{
if(ok)
return ;
if(x >= n)
{
if(!check())
{
printf("%s\n", s);
ok = 1;
}
return ;
}
if(s[x] >= 'a' && s[x] <= 'z')
{
dfs(x + 1);
return ;
}
else
{
for(char t = 'a'; t <= 'z'; t++)
{
s[x] = t;
dfs(x + 1);
s[x] = '?';
}
}
}
int main()
{
while(scanf("%d", &n)!=EOF)
{
scanf("%s", s);
ok = 0;
dfs(0);
if(!ok) printf("QwQ\n");
}
return 0;
}