hdu1402 A*B Problem Plus(FFT模板题)

板子来源

https://www.cnblogs.com/kuangbin/archive/2013/07/24/3210389.html//kuangbin巨神的板子

心得

写了一个从高位输出大数的方法，WA到不能自已，

后来发现是没判ans=0这种情况，才学会了kuangbin巨神的写法。

以后大数输出固定一下格式吧，感觉自己太菜了。

FFT模板get√ 生成函数什么的以后再刷

		for(int i=0;i<len;++i)
		{
			num[i+1]+=num[i]/10;
			num[i]%=10;
		}
		len=len1+len2-1;
		while(num[len]==0&&len>0)len--;
		for(int i=len;i>=0;--i)printf("%d",num[i]);

代码1（AC代码）

#include <stdio.h>
#include <iostream>
#include <string.h>
#include <algorithm>
#include <math.h>
using namespace std;

const double PI = acos(-1.0);
struct complex
{
    double r,i;
    complex(double _r = 0,double _i = 0)
    {
        r = _r; i = _i;
    }
    complex operator +(const complex &b)
    {
        return complex(r+b.r,i+b.i);
    }
    complex operator -(const complex &b)
    {
        return complex(r-b.r,i-b.i);
    }
    complex operator *(const complex &b)
    {
        return complex(r*b.r-i*b.i,r*b.i+i*b.r);
    }
};
void change(complex y[],int len)
{
    int i,j,k;
    for(i = 1, j = len/2;i < len-1;i++)
    {
        if(i < j)swap(y[i],y[j]);
        k = len/2;
        while( j >= k)
        {
            j -= k;
            k /= 2;
        }
        if(j < k)j += k;
    }
}
void fft(complex y[],int len,int on)
{
    change(y,len);
    for(int h = 2;h <= len;h <<= 1)
    {
        complex wn(cos(-on*2*PI/h),sin(-on*2*PI/h));
        for(int j = 0;j < len;j += h)
        {
            complex w(1,0);
            for(int k = j;k < j+h/2;k++)
            {
                complex u = y[k];
                complex t = w*y[k+h/2];
                y[k] = u+t;
                y[k+h/2] = u-t;
                w = w*wn;
            }
        }
    }
    if(on == -1)
        for(int i = 0;i < len;i++)
            y[i].r /= len;
}

const int MAXN = 400040;
complex x1[MAXN],x2[MAXN];
int num[MAXN];
char s1[MAXN],s2[MAXN];
int main()
{
    while(~scanf("%s%s",s1,s2))
    {
        memset(num,0,sizeof(num));
        int len1 = strlen(s1),len2=strlen(s2);
        int len = 1;
        while( len < len1*2 || len<len2*2 )len <<= 1;
        for(int i = 0;i < len1;i++)
            x1[i] = complex(s1[len1-i-1]-'0',0);
        for(int i = len1;i < len;i++)
            x1[i] = complex(0,0);
        for(int i = 0;i < len2;i++)
            x2[i] = complex(s2[len2-i-1]-'0',0);
        for(int i = len2;i < len;i++)
            x2[i] = complex(0,0);
        fft(x1,len,1);
        fft(x2,len,1);
        for(int i = 0;i < len;i++)
            x1[i] = x1[i]*x2[i];
        fft(x1,len,-1);
        for(int i = 0;i < len;i++)
            num[i] = (int)(x1[i].r+0.5);
		for(int i=0;i<len;++i)
		{
			num[i+1]+=num[i]/10;
			num[i]%=10;
		}
		len=len1+len2-1;
		while(num[len]==0&&len>0)len--;
		for(int i=len;i>=0;--i)printf("%d",num[i]);
        puts("");
    }
    return 0;
}

代码2（WA代码->AC代码）

#include <stdio.h>
#include <iostream>
#include <string.h>
#include <algorithm>
#include <math.h>
using namespace std;

const double PI = acos(-1.0);
struct complex
{
    double r,i;
    complex(double _r = 0,double _i = 0)
    {
        r = _r; i = _i;
    }
    complex operator +(const complex &b)
    {
        return complex(r+b.r,i+b.i);
    }
    complex operator -(const complex &b)
    {
        return complex(r-b.r,i-b.i);
    }
    complex operator *(const complex &b)
    {
        return complex(r*b.r-i*b.i,r*b.i+i*b.r);
    }
};
void change(complex y[],int len)
{
    int i,j,k;
    for(i = 1, j = len/2;i < len-1;i++)
    {
        if(i < j)swap(y[i],y[j]);
        k = len/2;
        while( j >= k)
        {
            j -= k;
            k /= 2;
        }
        if(j < k)j += k;
    }
}
void fft(complex y[],int len,int on)
{
    change(y,len);
    for(int h = 2;h <= len;h <<= 1)
    {
        complex wn(cos(-on*2*PI/h),sin(-on*2*PI/h));
        for(int j = 0;j < len;j += h)
        {
            complex w(1,0);
            for(int k = j;k < j+h/2;k++)
            {
                complex u = y[k];
                complex t = w*y[k+h/2];
                y[k] = u+t;
                y[k+h/2] = u-t;
                w = w*wn;
            }
        }
    }
    if(on == -1)
        for(int i = 0;i < len;i++)
            y[i].r /= len;
}

const int MAXN = 400040;
complex x1[MAXN],x2[MAXN];
int num[MAXN];
char s1[MAXN],s2[MAXN];
int main()
{
    while(~scanf("%s%s",s1,s2))
    {
        memset(num,0,sizeof(num));
        int len1 = strlen(s1),len2=strlen(s2);
        int len = 1;
        while( len < len1*2 || len<len2*2 )len <<= 1;
        for(int i = 0;i < len1;i++)
            x1[i] = complex(s1[len1-i-1]-'0',0);
        for(int i = len1;i < len;i++)
            x1[i] = complex(0,0);
        for(int i = 0;i < len2;i++)
            x2[i] = complex(s2[len2-i-1]-'0',0);
        for(int i = len2;i < len;i++)
            x2[i] = complex(0,0);
        fft(x1,len,1);
        fft(x2,len,1);
        for(int i = 0;i < len;i++)
            x1[i] = x1[i]*x2[i];
        fft(x1,len,-1);
        for(int i = 0;i < len;i++)
            num[i] = (int)(x1[i].r+0.5);
        bool flag=0;
		for(int i=0;i<len;++i)
		{
			num[i+1]+=num[i]/10;
			num[i]%=10;
		}
		len=len1+len2-1;
		for(int i=len;i>=0;--i)
		{
			if(num[i])flag=1;
			if(flag)printf("%d",num[i]);
		}
		if(!flag)printf("0");//加上这行就AC了，注意特判0
        puts("");
    }
    return 0;
}