Bit Strings

#include <bits/stdc++.h>
using namespace std;
int n, k;
const int M=1000000007;
long long dp0[1222333];
long long dp1[1222333];
int main(){
  cin >> n >> k;
  dp0[0]=0;
  dp1[0]=1;
  for (int i=1;i<=n;++i){
    dp0[i]=dp0[i-1]+dp1[i-1];
    if (dp0[i]>=M) dp0[i]-=M;
    dp1[i]=dp0[i-1]+dp1[i-1];
    if (dp1[i]>=M) dp1[i]-=M;
    if (i>=k) dp0[i]-=dp1[i-k];
    if (dp0[i]<0) dp0[i]+=M;
  }
  cout << (dp0[n]+dp1[n])%M << "\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