HDU 1402 FFT

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code :
#include <cstdio>#include <cmath>#include <cstring>using namespace std;typedef double ld;const ld PI = acos(-1.0);const int N = 50010;const int maxN = 150010;char A[maxN], B[maxN];inline int max(int a, int b){return a >= b ? a : b;}struct Complex{/**复数*/    ld real, vir;    Complex(){}    Complex(ld r, ld v):real(r), vir(v){}    Complex operator + (const Complex &rhs) const{        return Complex(real + rhs.real, vir + rhs.vir);    }    Complex operator - (const Complex &rhs) const{        return Complex(real - rhs.real, vir - rhs.vir);    }    Complex operator * (const Complex &rhs) const{        return Complex(real * rhs.real - vir * rhs.vir, real * rhs.vir + vir * rhs.real);    }}fa[maxN], fb[maxN], tmp[maxN];int ans[maxN];void FFT(Complex t[], int len, int f){/**递归写法*/    if(len == 1) return;    //f = -f;    Complex wn(cos(2 * PI / len * f), sin(2 * PI / len * f));    for(int i = 0; i < len / 2; ++i) tmp[i] = t[i * 2], tmp[len / 2 + i] = t[i * 2 + 1];    for(int i = 0; i < len; ++i) t[i] = tmp[i];    FFT(t, len / 2, f);    FFT(t + len / 2, len / 2, f);    Complex w(1, 0);    for(int i = 0; i < len / 2; ++i){        Complex u = t[i];        Complex v = w * t[i + len / 2];        t[i] = u + v;        t[i + len / 2] = u - v;        w = w * wn;    }}int main(){    while(~scanf("%s%s", A, B)){        memset(ans, 0, sizeof ans);        int lena = strlen(A), lenb = strlen(B);        int lent = max(lena, lenb);        lent <<= 1;        int len = 1;        while(len < lent) len <<= 1;/**2的幂*/       // printf("%d\n", len);        for(int i = 0; i < len; ++i){            if(i < lena) fa[i] = Complex(A[lena - i - 1] - '0', 0);            else fa[i] = Complex(0, 0);            if(i < lenb) fb[i] = Complex(B[lenb - i - 1] - '0', 0);            else fb[i] = Complex(0, 0);        }        FFT(fa, len, 1);/**系数到点值*/        FFT(fb, len, 1);        for(int i = 0; i < len; ++i) fa[i] = fa[i] * fb[i];        FFT(fa, len, -1);        for(int i = 0; i < len; ++i){            fa[i].real /= len;            fa[i].vir /= len;        }/**点值到系数*/        for(int i = 0; i < len; ++i) ans[i] = (int)(fa[i].real + 0.5);        for(int i = 0; i < len; ++i){            ans[i + 1] += ans[i] / 10;            ans[i] %= 10;        }        int id = lena + lenb - 1;        while(ans[id] == 0 && id) --id;/**注意0数据*/        for(int i = id; i >= 0; --i) printf("%d", ans[i]);        puts("");    }    return 0;}
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