String padding

input/code.cpp: In function 'int main()':
input/code.cpp:39:23: warning: array subscript has type 'char' [-Wchar-subscripts]
   39 |                     z[c][i] = j;
      |                       ^
input/code.cpp:49:27: warning: array subscript has type 'char' [-Wchar-subscripts]
   49 |                 int p = z[c][j];
      |                           ^

#include <bits/stdc++.h>
#define ll long long
#define ld long double
#define pb push_back
#define VI vector<int>
#define AR(x) array<int, x>
#define MX vector<VI >
#define all(x) x.begin(), x.end()
#define IOS cin.tie(0)->sync_with_stdio(0)

using namespace std;

const int P = 1e9+7;

int dp[1010][101];
int z[256][101];

int main(){

    int n;
    cin >> n;

    string s;
    cin >> s;

    int m = s.size();

    // dp[i][j] -> #valid strings
    // i -> length of string
    // j -> j last characters match prefix of forbidden string
    dp[0][0] = 1;

    for(int i=0; i<m; i++){
        for(char c='A'; c<='Z'; c++){
            string x = s.substr(0, i);
            x.pb(c);
            for(int j=min(s.size(), x.size()); j>=0; j--){
                if(x.substr(x.size()-j, j) == s.substr(0, j)){
                    z[c][i] = j;
                    break;
                }
            }
        }
    }

    for(int i=1; i<=n; i++){
        for(char c='A'; c<='Z'; c++){
            for(int j=0; j<m; j++){
                int p = z[c][j];
                dp[i][p] = (dp[i][p]+dp[i-1][j])%P;
            }
        }
    }

    int ans = 1;
    int d = 'Z'-'A'+1;
    for(int i=0; i<n; i++) ans = (ll)ans*d%P;

    for(int i=0; i<m; i++) ans = (ans-dp[n][i])%P;

    ans = (ans+P)%P;

    cout << ans << '\n';

    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED