用户:HermitOvO

这里是慕斯sama！~~（一直想改名为musesama，但是因为hermit的名称已经在各大论坛上了索性就用这个了）~~

会点中式建筑，有点红石技艺，浅尝辄止pvp，正在被带领学习中世纪建筑（

不善交际的美少女一枚捏（大概

出没地点：

各大群聊

记不住的东西：

算法知识点：

FFT 前方检测到核聚变打击，请谨慎打开！


#include <iostream>
#include <cstring>
#include <algorithm>
#include <cmath>

using namespace std;
const int N = 300000;
const double PI = acos(-1);
struct Complex
{

   double x, y;
   Complex operator+ (const Complex& t) const
   {
       return {x + t.x, y + t.y};
   }
   Complex operator- (const Complex& t) const
   {
       return {x - t.x, y - t.y};
   }
   Complex operator* (const Complex& t) const
   {
       return {x * t.x - y * t.y, x * t.y + y * t.x};
   }

}a[N], b[N];
char s1[N], s2[N];
int res[N];
int rev[N], bit, tot;
void fft(Complex a[], int inv)
{

   for (int i = 0; i < tot; i ++ )
       if (i < rev[i])
           swap(a[i], a[rev[i]]);
   for (int mid = 1; mid < tot; mid *= 2)
   {
       auto w1 = Complex({cos(PI / mid), inv * sin(PI / mid)});
       for (int i = 0; i < tot; i += mid * 2)
       {
           auto wk = Complex({1, 0});
           for (int j = 0; j < mid; j ++, wk = wk * w1)
           {
               auto x = a[i + j], y = wk * a[i + j + mid];
               a[i + j] = x + y, a[i + j + mid] = x - y;
           }
       }
   }

}
int main()
{

   scanf("%s%s", s1, s2);
   int n = strlen(s1) - 1, m = strlen(s2) - 1;
   for (int i = 0; i <= n; i ++ ) a[i].x = s1[n - i] - '0';
   for (int i = 0; i <= m; i ++ ) b[i].x = s2[m - i] - '0';
   while ((1 << bit) < n + m + 1) bit ++ ;
   tot = 1 << bit;
   for (int i = 0; i < tot; i ++ )
       rev[i] = ((rev[i >> 1] >> 1)) | ((i & 1) << (bit - 1));
   fft(a, 1), fft(b, 1);
   for (int i = 0; i < tot; i ++ ) a[i] = a[i] * b[i];
   fft(a, -1);

   int k = 0;
   for (int i = 0, t = 0; i < tot || t; i ++ )
   {
       t += a[i].x / tot + 0.5;
       res[k ++ ] = t % 10;
       t /= 10;
   }

   while (k > 1 && !res[k - 1]) k -- ;
   for (int i = k - 1; i >= 0; i -- ) printf("%d", res[i]);

   return 0;

}