2020牛客寒假算法基础集训营5 F-碎碎念
思路：
数位dp + 前缀和

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <queue>
#include <map>
#include <map>
#include <stack>
#include <set>

using namespace std;
typedef long long ll;
typedef pair<int,int>P;
const double eps = 1e-10;
const int INF = 0x3f3f3f3f;
const ll mod  = 1e9 + 7;
const ll maxn = 1e5 + 5;

ll f[maxn][2];
ll s[maxn];

int main(){
	int x,Q,L,R;
	cin>>x>>Q;
	f[0][0]=1;
	for(int i=1;i<=100000;i++){
		f[i][0]=(f[i-1][0]+f[i-1][1])%mod;
		if(i-x>=0) f[i][1]=f[i-x][0]%mod;
	}
	for(int i=1;i<=100000;i++){
		s[i]=(s[i-1]+f[i][0]+f[i][1])%mod;
	}
	while(Q--){
		cin>>L>>R;
		ll res=(mod+s[R]-s[L-1])%mod;
		cout<<res<<'\n';
	}
	return 0;
}