Bit Strings

#include <bits/stdc++.h>
#define ll long long
#define ld long double
#define ull unsigned long long
#define M 1000000007
#define N (1<<18)
#define P complex<long long>
#define X real()
#define Y imag()
 
using namespace std;

ll n, k, d[1010101][2], cntt;

void haku(int len, string s) {
	if(len == n) {
		char ed = 'G';
		int cnt = 0;
		for(auto u:s) {
			if(u == ed) {
				cnt++;
			} else {
				cnt = 1;
				ed = u;
			}
			if(cnt == k && ed == '0')
				return;
		}
		cntt++;
	}
	else {
		haku(len+1, s+"1");
		haku(len+1, s+"0");
	}
}

int main() {
	cin.tie(0);
	cout.tie(0);
	ios_base::sync_with_stdio(0);
	cin >> n >> k;
		if(k == 1) {
					cout << 1;
							exit(0);
								} else {
										
											}
												d[0][0] = 1;
													d[0][1] = (k==1)?0:1;
														d[1][0] = 1;
															d[1][1] = (k==1)?0:1;
	for(int i=2; i<=n; i++) {
		d[i][0] = d[i-1][0]+d[i-1][1];
		d[i][1] = d[i-1][0]+d[i-1][1];
		d[i][0] %= M;
		d[i][1] %= M;
		if(i-k>=0)
			d[i][0] -= d[i-k][1];
		d[i][0] %= M;
		d[i][0] += M;
		d[i][0] %= M;
		d[i][0] += M;
		d[i][0] %= M;
		d[i][1] %= M;
	}
	cout << (d[n][1]+d[n][0])%M;
}

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