hdu 5202 Rikka with string (dfs )

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

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

dfs:

 1 #include<cstdio>
 2 #include<cstring>
 3 using namespace std;
 4 int n,t,a[1005],flag,v[1005];
 5 char s[1005];
 6 int f()
 7 {
 8     int i;
 9     for (i=0;i<n/2;i++)
10     if (s[i]!=s[n-i-1]) return 0;
11     return 1;
12 }
13 void dfs(int p)
14 {
15    if (flag) return ;
16    if (p==t)
17    {
18        if (!f())
19        {
20            flag=1;
21            printf("%s
",s);
22        }
23        return ;
24    }
25    int i;
26    for (i='a';i<='z';i++)
27    {
28        if (!v[p])
29        {
30            v[p]=1;
31            s[a[p]]=i;
32            dfs(p+1);
33            v[p]=0;
34        }
35    }
36 }
37 int main()
38 {
39     int i;
40     while (~scanf("%d",&n))
41     {
42         t=0;
43         scanf("%s",&s);
44         for (i=0;i<n;i++)
45         {
46             if (s[i]=='?')
47             a[t++]=i;
48         }
49         memset(v,0,sizeof(v));
50         flag=0;
51         dfs(0);
52         if (!flag) printf("QwQ
");
53     }
54 }

 1 #include<cstdio>
 2 #include<cstring>
 3 using namespace std;
 4 char s[1005];
 5 int n;
 6 int f()
 7 {
 8     int i;
 9     for (i=0;i<n/2;i++)
10     if (s[i]!=s[n-i-1]) return 0;
11     return 1;
12 }
13 int main()
14 {
15     int i,a[1005],k;
16     while (~scanf("%d",&n))
17     {
18         k=0;
19         scanf("%s",&s);
20         for (i=0;i<n;i++)
21         {
22             if (s[i]=='?')
23             {
24                 s[i]='a';
25                 a[k++]=i;
26             }
27         }
28         if (n==0){printf("QwQ
");continue;}
29         if (k==0&&f()) {printf("QwQ
");continue;}
30         if (!f()) {printf("%s
",s);continue;}
31         s[a[k-1]]='b';
32         if (!f()) {printf("%s
",s);continue;}
33         if (k==1) {printf("QwQ
");continue;}
34         s[a[k-1]]='a';
35         s[a[k-2]]='b';
36         printf("%s
",s);
37     }
38    return 0;
39 }