FFT，还没有彻底理解，套了个板子。
#pragma comment(linker, "/STACK:1024000000,1024000000") #include<cstdio> #include<cstring> #include<cmath> #include<algorithm> #include<vector> #include<map> #include<set> #include<queue> #include<stack> #include<iostream> using namespace std; typedef long long LL; const double PI=acos(-1.0),eps=1e-8; void File() { freopen("D:\in.txt","r",stdin); freopen("D:\out.txt","w",stdout); } template <class T> inline void read(T &x) { char c = getchar(); x = 0;while(!isdigit(c)) c = getchar(); while(isdigit(c)) { x = x * 10 + c - '0'; c = getchar(); } } #define L(x) (1 << (x)) const int maxn = 200000; double ax[maxn], ay[maxn], bx[maxn], by[maxn]; int revv(int x, int bits) { int ret = 0; for (int i = 0; i < bits; i++) { ret <<= 1; ret |= x & 1; x >>= 1; } return ret; } void fft(double * a, double * b, int n, bool rev) { int bits = 0; while (1 << bits < n) ++bits; for (int i = 0; i < n; i++) { int j = revv(i, bits); if (i < j) swap(a[i], a[j]), swap(b[i], b[j]); } for (int len = 2; len <= n; len <<= 1) { int half = len >> 1; double wmx = cos(2 * PI / len), wmy = sin(2 * PI / len); if (rev) wmy = -wmy; for (int i = 0; i < n; i += len) { double wx = 1, wy = 0; for (int j = 0; j < half; j++) { double cx = a[i + j], cy = b[i + j]; double dx = a[i + j + half], dy = b[i + j + half]; double ex = dx * wx - dy * wy, ey = dx * wy + dy * wx; a[i + j] = cx + ex, b[i + j] = cy + ey; a[i + j + half] = cx - ex, b[i + j + half] = cy - ey; double wnx = wx * wmx - wy * wmy, wny = wx * wmy + wy * wmx; wx = wnx, wy = wny; } } } if (rev) { for (int i = 0; i < n; i++) a[i] /= n, b[i] /= n; } } int solve(int a[],int na,int b[],int nb,int ans[]) { int len = max(na, nb), ln; for(ln=0; L(ln)<len; ++ln); len=L(++ln); for (int i = 0; i < len ; ++i) { if (i >= na) ax[i] = 0, ay[i] =0; else ax[i] = a[i], ay[i] = 0; } fft(ax, ay, len, 0); for (int i = 0; i < len; ++i) { if (i >= nb) bx[i] = 0, by[i] = 0; else bx[i] = b[i], by[i] = 0; } fft(bx, by, len, 0); for (int i = 0; i < len; ++i) { double cx = ax[i] * bx[i] - ay[i] * by[i]; double cy = ax[i] * by[i] + ay[i] * bx[i]; ax[i] = cx, ay[i] = cy; } fft(ax, ay, len, 1); for (int i = 0; i < len; ++i) ans[i] = (int)(ax[i] + 0.5); return len; } char s[maxn],t[maxn]; int a[maxn],b[maxn],c[maxn],lena,lenb; int main() { while(~scanf("%s%s",s,t)) { memset(c,0,sizeof c); lena=strlen(s); lenb=strlen(t); if(lena==1&&s[0]=='0') {printf("0 "); continue;} if(lenb==1&&t[0]=='0') {printf("0 "); continue;} for(int i=lena-1;i>=0;i--) a[lena-1-i]=s[i]-'0'; for(int i=lenb-1;i>=0;i--) b[lenb-1-i]=t[i]-'0'; solve(a,lena,b,lenb,c); int k=0; for(int i=0;i<=100000;i++) { int p=c[i]+k; c[i]=p%10, k=p/10; } int len; for(int i=0;i<=100000;i++) if(c[i]!=0) len=i; for(int i=len;i>=0;i--) printf("%d",c[i]); printf(" "); } return 0; }