CodeForces 163A Substring and Subsequence

#include <bits/stdc++.h>
using namespace std;
const int maxn=5005;
const int MOD=1e9+7;
char a[maxn],b[maxn];
int dp[maxn][maxn];

int main()
{
    while(~scanf("%s%s",a+1,b+1)){
        int lena=strlen(a+1);
        int lenb=strlen(b+1);
        memset(dp,0,sizeof(dp));
        long long ans=0;
        for(int i=1;i<=lena;i++){
            for(int j=1;j<=lenb;j++){
                if(a[i]==b[j])dp[i][j]=(dp[i][j]+dp[i-1][j-1]+1)%MOD;
                dp[i][j]=(dp[i][j]+dp[i][j-1])%MOD;
            }
            ans=(ans+dp[i][lenb])%MOD;
        }
        printf("%lld\n",ans);
    }
    return 0;
}