Bit Strings

#include <bits/stdc++.h>
#define F first
#define S second
#define X real()
#define Y imag()
using namespace std;
typedef long long ll;
typedef long double ld;

ll as [1010101];
ll mod = 1000000007;

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    ll n, k;
    cin >> n >> k;
    as[0] = 1;
    for (ll i = 1; i < k; i++) {
         as[i] = ((ll)2*as[i - 1])%mod;
    }
    as[k] = ((ll)2*as[k - 1] - (ll)1)%mod;
    for (ll i = k; i < n; i++) {
        as[i + 1] = (2*as[i] - as[i - k] + mod)%mod;
    }
    cout << as[n] << "\n";
}

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