【洛谷P2822 组合数问题】

题目连接

#include<iostream>
#include<cstring>
#include<cstdio>
#include<cctype>
#define ll long long
#define gc() getchar()
#define maxn 2005
using namespace std;
inline ll read()
{
    ll a=0;int f=0;char p=gc();
    while(!isdigit(p)){f|=p=='-';p=gc();}
    while(isdigit(p)){a=(a<<3)+(a<<1)+(p^48);p=gc();}
    return f?-a:a;
}
void write(ll a)
{
    if(a>9)write(a/10);
    putchar(a%10+'0');
}

int t,k,c[maxn][maxn],f[maxn][maxn];
int main()
{
    t=read();k=read();
    for(int i=0;i<=2000;++i)c[i][0]=1;
    for(int i=1;i<=2000;++i)
        for(int j=1;j<=i;++j)
            c[i][j]=(c[i-1][j-1]+c[i-1][j])%k;
    for(int i=1;i<=2000;++i)
    {
        for(int j=1;j<=i;++j)
        {
            f[i][j]=f[i-1][j]+f[i][j-1]-f[i-1][j-1];
            if(!c[i][j])f[i][j]++;
        }
        f[i][i+1]=f[i][i];
    }
    for(int i=1;i<=t;++i)
    {
        int n=read(),m=read();
        if(m>n)m=n;
        write(f[n][m]);
        putchar('
');
    }
    return 0;
}

by子谦。（是HMY，不是钟子谦奆佬QWQ）大佬的面对面讲解