luogu[愚人节题目3]现代妖怪殖民地 NTT

U34272 [愚人节题目3]现代妖怪殖民地 fft

题目链接

https://www.luogu.org/problemnew/show/U34272

思路

虽然是个py题。
ntt(或者fft)模板题,可能稍不注意就会T

代码

#include <bits/stdc++.h>
using namespace std;
const int N=1e6+7,mod=998244353;
int n,m,a[N],b[N],limit=1,l,r[N];
int q_pow(int a,int b) {
    int ans=1;
    while(b) {
        if(b&1) ans=1LL*ans*a%mod;
        a=1LL*a*a%mod;
        b>>=1;
    }
    return ans;
}
void ntt(int *a,int type) {
    for(int i=0;i<limit;++i)
        if(i<r[i]) swap(a[i],a[r[i]]);
    for(int mid=1;mid<limit;mid<<=1) {
        int Wn=q_pow(3,(mod-1)/(mid<<1));
        for(int i=0;i<limit;i+=(mid<<1)) {
            for(int j=0,w=1;j<mid;++j,w=1LL*w*Wn%mod) {
                int x=a[i+j],y=1LL*w*a[i+j+mid]%mod;
                a[i+j]=(x+y)%mod;
                a[i+j+mid]=(x+mod-y)%mod;
            }
        }
    }
    if(type==-1) {
        reverse(&a[1],&a[limit]);
        int down=q_pow(limit,mod-2);
        for(int i=0;i<=limit;++i) a[i]=1LL*a[i]*down%mod;
    }
}
void solve() {
    limit=1,l=0,n=m=-1;
    char s=getchar();
    int flag=1;
    while(s==' '||s=='
') s=getchar();
    if(s=='-') flag*=-1;
    else a[++n]=s-'0';
    for(s=getchar();s>='0'&&s<='9';s=getchar()) a[++n]=s-'0';
    while(s==' '||s=='
') s=getchar();
    if(s=='-') flag*=-1;
    else b[++m]=s-'0';
    for(s=getchar();s>='0'&&s<='9';s=getchar()) b[++m]=s-'0';
    reverse(a,a+n+1);
    reverse(b,b+m+1);
    if(flag==-1) printf("-");
    while(limit<=n+m) limit<<=1,l++;
    for(int i=0;i<=limit;++i)
        r[i]=(r[i>>1]>>1)|((i&1)<<(l-1));
    ntt(a,1),ntt(b,1);
    for(int i=0;i<=limit;++i) a[i]=1LL*a[i]*b[i]%mod;
    ntt(a,-1);
    int len=limit;
    for(int i=0;i<=limit;++i) 
         a[i+1]+=a[i]/10,a[i]%=10;
    while(!a[limit]&&limit) limit--;
    while(limit>=0) printf("%d",a[limit--]);
    for(int i=0;i<=len;++i) a[i]=b[i]=0;
    printf("
");
}
int main() {
    int T;
    scanf("%d",&T);
    while(T--) solve();
    return 0;	
}