题解 | 小红的连续段

#include<bits/stdc++.h>
#define int unsigned long long

using namespace std;

const int N = 1005;
const int mod = 1e9+7;

inline int qpower(int x,int y){
    int res = 1;
    for(;y;x=x*x%mod, y>>=1) if(y&1) res = res*x%mod;
    return res;
}

int x, y;
int D[N],E[N];

inline int C(int x,int y){
    if(y < 0 || y > x) return 0;
    return D[x]*E[y]%mod*E[x-y]%mod;
}

signed main(){
    cin>>x>>y;
    D[0] = 1;
    D[1] = 1;
    for(int i=2;i<=1000;i++){
        D[i] = D[i-1] * i % mod;
    }
    for(int i=0;i<=1000;i++) E[i] = qpower(D[i], mod - 2)%mod;
    for(int i=1;i<=x+y;i++){
        int res = 0;
        int a1 = i/2, b1 = (i+1)/2;
        int a2 = (i+1)/2, b2 = i/2;
        res += C(x-1, a1-1)*C(y-1, b1-1)%mod;
        res %= mod;
        res += C(x-1, a2-1)*C(y-1, b2-1)%mod;
        res %= mod;
        cout<<res<<'\n';
    }
    return 0;
}