Codeforces 163A Substring and Subsequence

 1 #include<cstdio>
 2 #include<cmath>
 3 #include<cstring>
 4 #include<algorithm>
 5 #include<iostream>
 6 const int Mod=1000000007;
 7 char a[500005],b[500005];
 8 int n,m,f[5005][5005];
 9 int read(){
10      int t=0,f=1;char ch=getchar();
11      while (ch<‘0‘||ch>‘9‘){if (ch==‘-‘) f=-1;ch=getchar();}
12      while (‘0‘<=ch&&ch<=‘9‘){t=t*10+ch-‘0‘;ch=getchar();}
13      return t*f;
14 }
15 int main(){
16      scanf("%s",a+1);n=strlen(a+1);
17      scanf("%s",b+1);m=strlen(b+1);
18      for (int i=1;i<=n;i++)
19         if (a[i]==b[1]) f[i][1]=1;
20      for (int i=1;i<=m;i++)
21         if (a[1]==b[i]) f[1][i]=1;
22     int ans=0;
23     for (int i=1;i<=n;i++){
24         for (int j=1;j<=m;j++){
25            f[i][j]=f[i][j-1];
26            if (a[i]==b[j])
27                 (f[i][j]+=f[i-1][j-1]+1)%=Mod;
28         }
29         ans=(ans+f[i][m])%Mod;
30     }
31     printf("%d\n",ans);    
32 }