Codeforces 474D Flowers

http://codeforces.com/problemset/problem/474/D

思路:F[i]=F[i-1]+(i>=K)F[i-k]

#include<cstdio>
#include<cmath>
#include<algorithm>
#include<cstring>
#include<iostream>
const int Mod=1000000007;
int K,sum[200005],f[200005];
int read(){
    int t=0,f=1;char ch=getchar();
    while (ch<'0'||ch>'9'){if (ch=='-') f=-1;ch=getchar();}
    while ('0'<=ch&&ch<='9'){t=t*10+ch-'0';ch=getchar();}
    return t*f;
}
void init(){
    f[0]=1;
    for (int i=1;i<=100000;i++){
     f[i]=(f[i-1])%Mod;
     if (i>=K)
      f[i]=(f[i]+f[i-K])%Mod;
    }
    for (int i=1;i<=100000;i++)
     sum[i]=(sum[i-1]+f[i])%Mod; 
}
int main(){
    int T=read();K=read();
    init();
    while (T--){
        int l=read(),r=read();
        printf("%d
",((sum[r]-sum[l-1])%Mod+Mod)%Mod);
    }
    return 0;
}