Find the combination number AcWing 885. Find the combination number I

Original link

AcWing 885. Find the combination number I

Algorithm tags

Combinatorial mathematics Combination count Recurrence

Ideas

Code

#include<bits/stdc++.h>
#define int long long
#define rep(i, a, b) for(int i=a;i<b;++i)
#define Rep(i, a, b) for(int i=a;i>=b;--i)
using namespace std;
const int N = 2005, mod = 1e9+7;
int c[N][N];
inline int read(){
   int s=0,w=1;
   char ch=getchar();
   while(ch<'0'||ch>'9'){if(ch=='-')w=-1;ch=getchar();}
   while(ch>='0'&&ch<='9') s=s*10+ch-'0',ch=getchar();
   return s*w;
}
void put(int x) {
    if(x<0) putchar('-'),x=-x;
    if(x>=10) put(x/10);
    putchar(x%10^48);
}
void init(){
    rep(i, 0, N){
        rep(j, 0, i+1){
        	//  from i Select the 0 individual   The number of programmes is 1
            if(!j){
                c[i][j]=1;
            }else{
                c[i][j]=(c[i-1][j]+c[i-1][j-1])%mod;
            }
        }
    }
}
signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int n=read();
    init();
    while(n--){
        int a=read(), b=read();
        printf("%lld\n", c[a][b]);
    }
    return 0;
}

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