FFT模板(BZOJ2179)

#include <cstdio>
#include <cmath>
#include <complex>
using namespace std;
#define pi acos(-1)

typedef complex<double> C;
const int N = 131100;
char s
,t
;
int n,m,l,r
,c
;
C a
,b
;

void fft(C *a, int f) {
for(int i = 0; i < n; i++) if(r[i] > i) swap(a[i], a[r[i]]);
for(int i = 1; i < n; i <<= 1) {
C wn(cos(pi/i), f*sin(pi/i));
for(int j = 0; j < n; j += i<<1) {
C w = 1;
for(int k = 0; k < i; k++, w *= wn) {
C x = a[j+k], y = w*a[j+k+i];
a[j+k] = x+y, a[j+k+i] = x-y;
}
}
}
}

int main() {
scanf("%d%s%s", &m, s, t);
for(int i = 0; i < m; i++) a[i] = s[m-i-1]-'0', b[i] = t[m-i-1]-'0';
for(n = 1, m <<= 1; n < m; n <<= 1) l++;
for(int i = 0; i < n; i++) r[i] = (r[i>>1]>>1)|((i&1)<<(l-1));
fft(a, 1), fft(b, 1);
for(int i = 0; i < n; i++) a[i] *= b[i];
fft(a, -1);
for(int i = 0; i < n; i++) a[i] /= n;
for(int i = 0; i < m; i++) c[i] = (int)(a[i].real()+0.1);
for(int i = 0; i < m; i++) if(c[i] >= 10) {
c[i+1] += c[i]/10, c[i] %= 10;
} else if(!c[i] && i == m-1) m--;
for(int i = m-1; ~i; i--) printf("%d", c[i]);
return 0;
}