求这个完整的前面密文后面明文的串 

# include <stdio.h>
# include <algorithm>
# include <string.h>
using namespace std;
int len;
int next[100010];
char a1[100010],a2[100010],a3[100010];
void Getnext()
{
    int i=0,j=-1;
    next[0]=-1;
    while(i<len)
    {
        if(j==-1||a2[j]==a2[i])
           i++,j++,next[i]=j;
        else
            j=next[j];
    }
}
int KMP()
{
    int len1=strlen(a3);
    int len2=strlen(a2);
    int i=0,j=0;
    while(i<len1&&j<len2)
    {
        if(j==-1||a2[j]==a3[i])
        {
            i++;j++;
            if(i==len1)
                return j;
        }
        else
            j=next[j];
    }
    return 0;

}
int main()
{
    int i,j,t;
    while(~scanf("%d",&t))
    {
        while(t--)
        {
            scanf("%s %s",a1,a2);
            printf("%s",a2);
            len=strlen(a2);
            strcpy(a3,a2+(len+1)/2);
            for(i=0;a2[i];i++)//转化为加密串
            {
                for(j=0;j<26;j++)
                {
                    if(a2[i]==a1[j])
                    {
                        a2[i]='a'+j;
                        break;
                    }
                }
            }
            for(i=0;a2[i];i++)
                printf("%c",a2[i]);
            Getnext();//a2

            int flag=KMP();//a3,a2
            for(i=flag;i<len-flag;i++)
                printf("%c",a2[i]);
            printf("\n");

        }
    }
    return 0;
}