D. Flowers

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

题意:土拔鼠可以吃红或白花,但白花只能在连续k个白花时才吃百花(一开始没读出这句话=-=);n个询问a,b问a~b朵花的序列中有多少种吃法;

思路:dp;dp[i]=dp[i-1]+dp[i-k];打表;然后求前缀和;

总结:sum[b]-sum[a-1]没有加mod再mod会出现负数!!

#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
using namespace std;

#define sfi(i) scanf("%d",&i)
#define pri(i) printf("%d\n",i)
#define sff(i) scanf("%lf",&i)
#define ll long long
#define mem(x,y) memset(x,y,sizeof(x))
#define INF 0x3f3f3f3f
#define eps 1e-6
#define PI acos(-1)
#define lowbit(x) ((x)&(-x))
#define zero(x) (((x)>0?(x):-(x))
inline void sc(T &ret)
{
    char c;
    ret = 0;
    while ((c = getchar()) < '0' || c > '9');
    while (c >= '0' && c <= '9')
    {
        ret = ret * 10 + (c - '0'), c = getchar();
    }
}

ll dp[maxn];
ll sum[maxn];
int main()
{
    int n,k;
    cin>>n>>k;
    dp[0]=1;
    sum[0]=0;
    for(int i=1;i<=maxn;i++)
    {
        dp[i]+=dp[i-1];
        dp[i]%=mod;
        if(i>=k)
        {
            dp[i]+=dp[i-k];
            dp[i]%=mod;
        }
        sum[i]=sum[i-1]+dp[i];
        sum[i]%=mod;
    }
    while(n--)
    {
        int a,b;
        cin>>a>>b;
        cout<<(sum[b]-sum[a-1]+mod)%mod<

 

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