【HDOJ6630】permutation 2（递推）

题意：给定x，y，n，有标号从1到n的n个数组，求合法的排列个数模998244353使得

1：p[1]=x

2：p[n]=y

3：相邻两项的差的绝对值<=2

n<=1e5

思路：

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 typedef long long ll;
 4 typedef unsigned int uint;
 5 typedef unsigned long long ull;
 6 typedef pair<int,int> PII;
 7 typedef pair<ll,ll> Pll;
 8 typedef vector<int> VI;
 9 typedef vector<PII> VII;
10 #define N  1100000
11 #define M  4100000
12 #define fi first
13 #define se second
14 #define MP make_pair
15 #define pi acos(-1)
16 #define mem(a,b) memset(a,b,sizeof(a))
17 #define rep(i,a,b) for(int i=(int)a;i<=(int)b;i++)
18 #define per(i,a,b) for(int i=(int)a;i>=(int)b;i--)
19 #define lowbit(x) x&(-x)
20 #define Rand (rand()*(1<<16)+rand())
21 #define id(x) ((x)<=B?(x):m-n/(x)+1)
22 #define ls p<<1
23 #define rs p<<1|1
24 
25 const ll MOD=998244353,inv2=(MOD+1)/2;
26       double eps=1e-6;
27       int INF=1e9;
28 
29 int f[110000];
30 
31 int read()
32 {
33    int v=0,f=1;
34    char c=getchar();
35    while(c<48||57<c) {if(c=='-') f=-1; c=getchar();}
36    while(48<=c&&c<=57) v=(v<<3)+v+v+c-48,c=getchar();
37    return v*f;
38 }
39 
40 
41 
42 int main()
43 {
44     int cas=read();
45     f[1]=f[2]=f[3]=1;
46     rep(i,4,100000) f[i]=(f[i-1]+f[i-3])%MOD;
47     while(cas--)
48     {
49         int n=read(),x=read(),y=read();
50         if(x>1) x++;
51         if(y<n) y--;
52         printf("%d\n",f[y-x+1]);
53     }
54 
55     return 0;
56 }