[hdu1402]大数乘法(FFT模板)

题意：大数乘法

思路：FFT模板

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/* ******************************************************************************** */ #include <iostream>                                                                 // #include <cstdio>                                                                   // #include <cmath>                                                                    // #include <cstdlib>                                                                  // #include <cstring>                                                                  // #include <vector>                                                                   // #include <ctime>                                                                    // #include <deque>                                                                    // #include <queue>                                                                    // #include <algorithm>                                                                // #include <map>                                                                      // #include <cmath>                                                                    // using namespace std;                                                                //                                                                                     // #define pb push_back                                                                // #define mp make_pair                                                                // #define X first                                                                     // #define Y second                                                                    // #define all(a) (a).begin(), (a).end()                                               // #define foreach(a, i) for (typeof(a.begin()) i = a.begin(); i != a.end(); ++ i)     // #define foreach(a, n, i) for(typeof(*a) *i = a; i < a + n; i ++)                    // #define fillchar(a, x) memset(a, x, sizeof(a))                                      //                                                                                     // void RI(vector<int>&a,int n){a.resize(n);for(int i=0;i<n;i++)scanf("%d",&a[i]);}    // void RI(){}void RI(int&X){scanf("%d",&X);}template<typename...R>                    // void RI(int&f,R&...r){RI(f);RI(r...);}void RI(int*p,int*q){int d=p<q?1:-1;          // while(p!=q){scanf("%d",p);p+=d;}}void print(){cout<<endl;}template<typename T>      // void print(const T t){cout<<t<<endl;}template<typename F,typename...R>              // void print(const F f,const R...r){cout<<f<<", ";print(r...);}template<typename T>   // void print(T*p, T*q){int d=p<q?1:-1;while(p!=q){cout<<*p<<", ";p+=d;}cout<<endl;}   //                                                                                     // typedef pair<int, int> pii;                                                         // typedef long long ll;                                                               // typedef unsigned long long ull;                                                     //                                                                                     // template<typename T>bool umax(T&a, const T&b){return b>a?false:(a=b,true);}         // template<typename T>bool umin(T&a, const T&b){return b<a?false:(a=b,true);}         // template<typename T>                                                                // void V2A(T a[],const vector<T>&b){for(int i=0;i<b.size();i++)a[i]=b[i];}            // template<typename T>                                                                // void A2V(vector<T>&a,const T b[]){for(int i=0;i<a.size();i++)a[i]=b[i];}            //                                                                                     // const double PI = acos(-1);                                                         //                                                                                     // /* -------------------------------------------------------------------------------- */   namespace FFT {     const static int maxn = 5e4 + 7;     #define L(x) (1 << (x))     double ax[maxn << 2], ay[maxn << 2], bx[maxn << 2], by[maxn << 2];//需要四倍空间     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;     }     #undef L(x) } const int maxn = 5e4 + 7; char s1[maxn], s2[maxn]; int x[maxn], y[maxn], ans[maxn << 2];   int main() { #ifndef ONLINE_JUDGE     freopen("in.txt", "r", stdin); #endif // ONLINE_JUDGE     while (~scanf("%s", s1)) {         scanf("%s", s2);         int len1 = strlen(s1), len2 = strlen(s2);         for (int i = 0; i < len1; i ++) x[i] = s1[len1 - i - 1] - '0';         for (int i = 0; i < len2; i ++) y[i] = s2[len2 - i - 1] - '0';         fillchar(ans, 0);         int len = FFT::solve(x, len1, y, len2, ans), i;         for (i = 0; i < len || ans[i] >= 10; i ++) {             ans[i + 1] += ans[i] / 10;             ans[i] %= 10;         }         len = i;         while (ans[len] <= 0 && len) len --;         for (int i = len; i >= 0; i --) putchar(ans[i] + '0');         puts("");     }     return 0; } /* ******************************************************************************** */