Required_Substring.cpp

#include <bits/stdc++.h>
using namespace std;
// First you make it work, then you can always make it beautiful
#define int long long
const int mod = 1e9 + 7;

int modpowr(int a, int b) {
    int res = 1;
    while(b) {
        if(b & 1) res = (res * a) % mod;
        a = (a * a) % mod;
        b >>= 1;
    }
    return res;
}
// dp[i][j] = number of ways to build a string of length n
// given that we are currently at position i
// and have matched j characters of the pattern s as a suffix.
int32_t main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);

    int n;
    cin >> n;
    string s;
    cin >> s;
    int m = s.size();

    vector<int> lps(m);
    for(int i=1;i<m;i++) {
        int prev = lps[i-1];
        while(prev > 0 && s[i] != s[prev]) {
            prev = lps[prev - 1];
        }
        lps[i] = prev + (s[i] == s[prev]);
    }

    vector<vector<int>> dp(n + 1, vector<int>(m + 1));

    for(int i=0;i<=n;i++) {
        dp[i][m] = modpowr(26, n - i);
    }

    for(int i=n-1;i>=0;i--) {
        for(int j=0;j<m;j++) {
            for(int k=0;k<26;k++) {
                int idx = j;
                while(idx > 0 && k != s[idx] - 'A') {
                    idx = lps[idx - 1];
                }
                if(k == s[idx] - 'A') idx++;
                (dp[i][j] += dp[i+1][idx]) %= mod;
            }
        }
    }

    int ans = dp[0][0];
    cout << ans << "\n";

    return 0;
}