高精度运算之乘法

乘法运算之终极状态。（可以处理所有乘法运算，略叼）。

#include<stdio.h>
#include<iostream>
#include<string>
#include<algorithm>
#include<string.h>
using namespace std;
string add(string st1,string st2)
{
    if(st1.size()<st2.size())           //判断哪个长
    {
        string stemp=st1;
        st1=st2;
        st2=stemp;
    }
    reverse(st1.begin(),st1.end());         //将string类st1,st2逆转顺序
    reverse(st2.begin(),st2.end());
    int len1,len2;
    len1=st1.length();                      //计算各自的长度
    len2=st2.length();
    int in=0,num;                           //进位in初始化为0
    for(int i=0; i<len2; i++)
    {
        num=st1[i]-'0'+st2[i]-'0'+in;       //各位的和
        in=num/10;
        num%=10;
        st1[i]=num+'0';                     //放入较长的string里
    }
    for(int j=len2; j<len1; j++)            //如果遇到例如123 99892这样的样例,123处理完后需要向前边进位
    {
        num=st1[j]-'0'+in;
        in=num/10;
        num%=10;
        st1[j]=num+'0';
    }
    if(in==1)                               //相加的结果比原有的string都长
        st1+="1";
    reverse(st1.begin(),st1.end());
    return st1;
}
string multiplication(string st1,string st2)
{
    if(st1[0]=='0'||st2[0]=='0')
        return "0";
    reverse(st1.begin(),st1.end());
    reverse(st2.begin(),len2,len;
    len1=st1.length(),len2=st2.length();
    if(len1<len2)
    {
        string stemp=st1;
        st1=st2;
        st2=stemp;
    }
    len1=st1.size();
    len=len2=st2.length();
    string stnum;
    for(int i=0; i<len; i++)
    {
        string stemp;
        for(int k=0; k<i; k++)
        {
            stemp+="0";
        }
        int muti,in=0;
        for(int j=0; j<len1; j++)
        {
            muti=(st2[i]-'0')*(st1[j]-'0')+in;
            in=muti/10;
            muti=muti%10;
            stemp+=muti+'0';
        }
        if(in>0)
            stemp+=in+'0';
        reverse(stemp.begin(),stemp.end());
        //   cout<<stemp<<endl;
        stnum= add(stnum,stemp);
    }
    return stnum;
}
string flo_multi(string ss1,string ss2)
{
    int len1,pos_s1=0,pos_s2=0,num_f1=0,num_f2=0;
    string str1,str2;
    len1=ss1.length(),len2=ss2.length();
    bool flag1=0,flag2=0,flag=0;
    for(int i=len1-1; i>=0; i--)
    {
        if(ss1[i]=='.')
        {
            pos_s1=i;
            break;
        }
        if(ss1[i]=='0'&&flag1==0)
        {
            continue;
        }
        flag1=1;
        str1+=ss1[i];
    }
    int las_len1=str1.length();
 //   cout<<las_len1<<endl;
    reverse(str1.begin(),str1.end());
    //  cout<<str1<<endl;
    flag1=0;
    string tmp1;
    for(int i=0; i<pos_s1; i++)
    {
        flag=1;
        if(ss1[i]=='0'&&flag1==0)
        {
            continue;
        }
        flag1=1;
        tmp1+=ss1[i];
    }
    str1=tmp1+str1;
    if(!flag)
    {
        num_f1=0;
    }
    else
    {
        num_f1=las_len1;
    }
    flag=0;


    //  cout<<str1<<endl;
    for(int i=len2-1; i>=0; i--)
    {
        if(ss2[i]=='.')
        {
            pos_s2=i;
            break;
        }
        if(ss2[i]=='0'&&flag2==0)
        {
            continue;
        }
        flag2=1;
        str2+=ss2[i];
    }
  //  cout<<str2<<endl;
    int las_len2=str2.length();
    flag2=0;
    reverse(str2.begin(),str2.end());
  //  cout<<str2<<endl;
    string tmp2;
    for(int i=0; i<pos_s2; i++)
    {
        flag=1;
        if(ss2[i]=='0'&&flag2==0)
        {
            continue;
        }
        flag2=1;
        tmp2+=ss2[i];
    }
    str2=tmp2+str2;
  //  cout<<str2<<endl;
    if(!flag)
    {
        num_f2=0;
    }
    else
    {
        num_f2=las_len2;
    }
//    cout<<num_f1<<" "<<num_f2<<endl;
    if(flag1==0&&num_f1!=0)
    {
        string temp1;
        reverse(str1.begin(),str1.end());
        for(int i=num_f1-1; i>=0; i--)
        {
            if(str1[i]=='0'&&flag1==0)
            {
                continue;
            }
            flag1=1;
            temp1+=str1[i];
        }
        str1=temp1;
        //   cout<<str1<<endl;
    }
     // cout<<flag2<<endl<<num_f2<<endl;
    if(flag2==0&&num_f2!=0)
    {
        string temp2;
        reverse(str2.begin(),str2.end());
    //    cout<<str2<<endl;
        for(int i=num_f2-1; i>=0; i--)
        {
            if(str2[i]=='0'&&flag2==0)
            {
                continue;
            }
            flag2=1;
            temp2+=str2[i];
        }
        str2=temp2;
        //   cout<<str2<<endl;
    }
  //  cout<<str1.length()<<endl<<str2.length()<<endl;
    if(str1.length()==0)
    {
        str1="0";
    }
    if(str2.length()==0)
    {
        str2="0";
    }
 //  cout<<str1<<endl<<str2<<endl;

    string ans_st= multiplication(str1,str2);
    string ans;
 //   cout<<ans_st<<endl;
//    cout<<ans_st.length()<<endl<<num_f1+num_f2<<endl;
    if(ans_st.length()>num_f1+num_f2&&(pos_s1!=0||pos_s2!=0))
    {
        reverse(ans_st.begin(),ans_st.end());
        for(int i=0; i<num_f1+num_f2; i++)
        {
            ans+=ans_st[i];
        }
        ans+='.';
        for(int i=num_f1+num_f2; i<ans_st.length(); i++)
        {
            ans+=ans_st[i];
        }
        reverse(ans.begin(),ans.end());
    }
    else if(ans_st.length()<=num_f1+num_f2&&(pos_s1!=0||pos_s2!=0))
    {
        reverse(ans_st.begin(),ans_st.end());
        for(int i=0; i<ans_st.length(); i++)
        {
            ans+=ans_st[i];
        }
        for(int i=0; i<num_f1+num_f2-ans_st.length(); i++)
        {
            ans+='0';
        }
        ans+='.';
        ans+='0';
        reverse(ans.begin(),ans.end());
    }
    else
    {
        ans=ans_st;
    }
    // cout<<ans<<endl;
    return ans;
}
int main()
{
    // freopen("INput.txt","r",stdin);
    // freopen("OUT.txt","w",stdout);
    string s1,s2;
    while(cin>>s1>>s2)
    {
        cout<<flo_multi(s1,s2)<<endl;
    }
    return 0;
}