Submit: 1626 Solved: 994
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Description
Input
Output
输出 T 行,每行一个数,表示求出的序列数
Sample Input
1 0
1 1
5 2
100 50
10000 5000
Sample Output
1
20
578028887
60695423
HINT
Source
#include<cstdio> #define int long long using namespace std; const int mod=1e9+7; const int MAXN=1e6+10; inline int read() { char c=getchar();int x=0,f=1; while(c<'0'||c>'9'){if(c=='-')f=-1;c=getchar();} while(c>='0'&&c<='9'){x=x*10+c-'0';c=getchar();} return x*f; } int fac[MAXN],D[MAXN],inv[MAXN]; void Pre() { fac[0]=fac[1]=inv[0]=inv[1]=D[0]=D[2]=1; for(int i=2;i<=1000001;i++) fac[i]=(i*fac[i-1])%mod; for(int i=2;i<=1000001;i++) inv[i]=(mod-mod/i)*inv[mod%i]%mod; for(int i=2;i<=1000001;i++) inv[i]=(inv[i]*inv[i-1])%mod; for(int i=3;i<=1000001;i++) D[i]=((i-1)*(D[i-1]+D[i-2]))%mod; } int Query(int N,int M) { return fac[N] %mod * inv[ N-M ] %mod * inv[ M ] %mod * D[N-M] %mod; } main() { Pre(); int T=read(); while(T--) { int N=read(),M=read(); printf("%lld\n",Query(N,M)%mod); } return 0; }