hdu 5202 Rikka with string (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\n",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\n");
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\n");continue;}
29         if (k==0&&f()) {printf("QwQ\n");continue;}
30         if (!f()) {printf("%s\n",s);continue;}
31         s[a[k-1]]=‘b‘;
32         if (!f()) {printf("%s\n",s);continue;}
33         if (k==1) {printf("QwQ\n");continue;}
34         s[a[k-1]]=‘a‘;
35         s[a[k-2]]=‘b‘;
36         printf("%s\n",s);
37     }
38    return 0;
39 }