Keep at Most 100 Characters

Given a string which contains only lower case letters, how many different non-empty strings you can get if you can keep AT MOST 100 characters in the original order? (Note: The string obtained is a "sub-sequence" of the original string.)

Input Specification:

Each input file contains one test case, which is only one line containing the string whose length is no larger than 1000.

Output Specification:

Output the number of different non-empty strings if you can only keep at most 100 characters. Since the answer might be super large, you only need to output the answer modulo 1000000007.

Sample Input:

aabac

 

Sample Output:

19

 

Hint:

The strings are:

a, b, c, aa, ab, ac, ba, bc, aab, aaa, aac, aba, abc, bac, aaba, aabc, abac, aaac, and aabac.

#include<bits/stdc++.h>
using namespace std;
#define mod 1000000007u
int main()
{
//    freopen("data.txt","r",stdin);
    char a[1001];
    scanf("%s",a);
    int asl=strlen(a)+1;
    long long op=0;
    int masl=min(asl,101);
    vector<long long> v(asl*masl,1);
    for(int i=1;i<masl;i++)
    {
        vector<long long> maxre(26,0);
        maxre[a[i-1]-'a']=1;
        for(int j=i+1;j<asl;j++)
        {
            v[i*asl+j]=(v[i*asl+j-1]+v[(i-1)*asl+j-1]-maxre[a[j-1]-'a']+mod)%mod;
            maxre[a[j-1]-'a']=v[(i-1)*asl+j-1];
        }
    }
    for(int i=1;i<masl;i++)
    op=(op+v[(i+1)*asl-1])%mod;    
    printf("%lld",op);
    return 0;
}