Bit Strings

#include <iostream>
using namespace std;
using ll = long long;

const int N = 1e6 + 5;
const int M = 1e9 + 7;

ll pow[N];
ll g[N];
ll f[N];

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(0);
        ll n, k;
        cin >> n >> k;
        pow[0] = 1;
        for(int i = 1; i < N; ++i) {
            pow[i] = pow[i-1]*2;
            pow[i] %= M;
        }
        g[0] = 0;
        for(int i = 1; i <= n; ++i) {
            //Calc g[i]
            if(i - k + 1 < 3) {
                g[i] = 0;
            } else {
                g[i] = 2*g[i] + f[i - k];
            }
            g[i] += M;
            g[i] %= M;
            //Calc f[i]
            if(i < k) {
                f[i] = pow[i];
                continue;
            }
            if(i == k) {
                f[i] = pow[i] - 1;
                continue;
            }
            f[i] = pow[i] - pow[i - k] - pow[i-k-1] - g[i];
            f[i] += M;
            f[i] %= M;
            
        }
        if(f[n] < 0) f[n] += M;
        cout << f[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: WRONG ANSWER

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: WRONG ANSWER

Test 12

Verdict: WRONG ANSWER

Test 13

Verdict: WRONG ANSWER

Test 14

Verdict: WRONG ANSWER

Test 15

Verdict: WRONG ANSWER

Test 16

Verdict: WRONG ANSWER

Test 17

Verdict: WRONG ANSWER

Test 18

Verdict: ACCEPTED