luogu P1919 【模板】A*B Problem升级版（FFT快速傅里叶）

模板
嗯
做多项式乘法,进位
没了

#include<cmath>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define pi acos(-1.0)
inline int read() {
    int x = 0,f = 1;
    char c = getchar();
    while(c < '0' || c > '9')c = getchar();
    while(c <= '9' && c >= '0')x = x * 10 + c - '0',c = getchar();
    return x * f;
}
const int maxn = 400007;
struct cp {
    double x,y;
    cp (double a = 0,double b = 0): x(a),y(b) {};
} A[maxn],B[maxn];
cp operator + (cp a,cp b) { return cp(a.x + b.x,a.y + b.y);}
cp operator - (cp a,cp b) { return cp(a.x - b.x,a.y - b.y);}
cp operator * (cp a,cp b) { return cp(a.x * b.x - a.y * b.y,a.x * b.y + a.y * b.x); }
void fft(cp *a,int n, int type) {
    for(int i = 0,j = 0;i < n;++ i)  {
        if(i < j) std::swap(a[i],a[j]);
            for(int k = n >> 1;(j ^= k) < k;k >>= 1);
    }
    for(int m = 2;m <= n;m <<= 1) {
        cp w1 = cp(cos(2 * pi / m),type * sin(2 * pi / m));
        for(int i = 0;i < n;i += m) {
            cp w = cp(1.0,0);
            for(int k = 0;k < (m >> 1);++ k) {
                cp t = w * a[i + k + (m >> 1)],u = a[i + k];
                a[i + k] = u + t;
                a[i + k + (m >> 1)] = u - t;
                w = w * w1;
            }
        }
    }
}
int c[maxn];
char s1[maxn],s2[maxn];
int main() {
    int n = read(); --n;
    scanf("%s%s",s1,s2);
    for(int i  = 0;i <= n;++ i) A[i].x = s1[n - i] - '0',B[i].x = s2[n -i] - '0';
    int tmp = n;
    //printf("%d\n",n);
    for(n = 1;n <= tmp * 2;n <<= 1);
    //printf("%d\n",n);
    fft(A,n,1);
    fft(B,n,1);
    /*for(int i = 0;i <= n;++ i)  {
        printf("%lf\n",A[i].x);
    } */
    for(int i = 0;i <= n;++ i) A[i] = A[i] * B[i];
    fft(A,n,-1);
    for(int i = 0;i <= n;++ i) c[i] = int(A[i].x / n + 0.5);
    for(int i = 0;i <= n;++ i)
        if(c[i] > 10) {
            //printf("%d\n",c[i]);
            c[i + 1] += c[i] / 10,c[i] %= 10;
            if(i + 1 > n) n ++;
        }
    for(int i = n;i >= 0;i --)
        if(!c[i]) n--;else break;
    for(int i = n;i >= 0;i --) printf("%d",c[i]);
    puts("");
    return 0;
}
 

原文地址：https://www.cnblogs.com/sssy/p/9192085.html