（模板）高精度算法

高精度加法，减法，乘法，通过重载运算符及自定义数据类型实现；

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;
const int N=4005;
char A[N],B[N];
struct data{
    int len;
    int v[N];
    data(){
        memset(v,0,sizeof(v));
    }
    void print(){
        for(int i=len;i;i--)
            cout<<v[i];
        cout<<endl;
    }
};
bool operator<(data a,data b)
{
    if(a.len<b.len)return true;
    if(a.len>b.len)return false;
    for(int i=a.len;i;i--){
        if(a.v[i]<b.v[i])return true;
        if(a.v[i]>b.v[i])return false;
    }
    return false;
}
data operator+(data a,data b)
{
    data ans;
    ans.len=max(a.len,b.len);
    for(int i=1;i<=ans.len;i++)
        ans.v[i]=a.v[i]+b.v[i];
    for(int i=1;i<=ans.len;i++){
        ans.v[i+1]+=ans.v[i]/10;
        ans.v[i]%=10;
    }
    if(ans.v[ans.len+1])ans.len++;
    return ans;
}
data operator-(data a,data b)//a>b!!!
{
    data ans;
    ans.len=a.len;
    for(int i=1;i<=a.len;i++){
        if(a.v[i]<b.v[i])a.v[i]+=10,a.v[i+1]--;
        ans.v[i]=a.v[i]-b.v[i];
    }
    while(!ans.v[ans.len]&&ans.len>1)ans.len--;
    return ans;
}
data operator*(data a,data b)
{
    data ans;
    ans.len=a.len+b.len;
    for(int i=1;i<=a.len;i++)
        for(int j=1;j<=b.len;j++)
            ans.v[i+j-1]+=a.v[i]*b.v[j];
    for(int i=1;i<=ans.len;i++){
        ans.v[i+1]+=ans.v[i]/10;
        ans.v[i]%=10;
    }
    while(!ans.v[ans.len]&&ans.len>1)ans.len--;//注意必须加上ans.len>1,否则有一个数为0时会re

    return ans;
}
int main()
{
    //gets(A+1);gets(B+1);
    //cin.getline(A+1,sizeof(A));cin.getline(B+1,sizeof(B));
    scanf("%s",A+1);scanf("%s",B+1);//几种读取方法
    data a,b,c;
    a.len=strlen(A+1);b.len=strlen(B+1);
    for(int i=1;i<=a.len;i++)
        a.v[i]=A[a.len-i+1]-'0';
    for(int i=1;i<=b.len;i++)
        b.v[i]=B[b.len-i+1]-'0';
//    if(a<b){
//        c=a;a=b;b=c;cout<<'-';//如果是减法必须把两个数交换再减
//    }
//    c=a-b;
    c=a*b;
    c.print();

    return 0;
}

另一个看起来更完善的模板：

#include <iostream>
#include <string>
#include <cstring>
#include <cstdio>
using namespace std;

const int maxn = 1000;

struct bign{
    int d[maxn], len;

    void clean() { while(len > 1 && !d[len-1]) len--; }

    bign()          { memset(d, 0, sizeof(d)); len = 1; }
    bign(int num)   { *this = num; } 
    bign(char* num) { *this = num; }
    bign operator = (const char* num){
        memset(d, 0, sizeof(d)); len = strlen(num);
        for(int i = 0; i < len; i++) d[i] = num[len-1-i] - '0';
        clean();
        return *this;
    }
    bign operator = (int num){
        char s[20]; sprintf(s, "%d", num);
        *this = s;
        return *this;
    }

    bign operator + (const bign& b){
        bign c = *this; int i;
        for (i = 0; i < b.len; i++){
            c.d[i] += b.d[i];
            if (c.d[i] > 9) c.d[i]%=10, c.d[i+1]++;
        }
        while (c.d[i] > 9) c.d[i++]%=10, c.d[i]++;
        c.len = max(len, b.len);
        if (c.d[i] && c.len <= i) c.len = i+1;
        return c;
    }
    bign operator - (const bign& b){
        bign c = *this; int i;
        for (i = 0; i < b.len; i++){
            c.d[i] -= b.d[i];
            if (c.d[i] < 0) c.d[i]+=10, c.d[i+1]--;
        }
        while (c.d[i] < 0) c.d[i++]+=10, c.d[i]--;
        c.clean();
        return c;
    }
    bign operator * (const bign& b)const{
        int i, j; bign c; c.len = len + b.len; 
        for(j = 0; j < b.len; j++) for(i = 0; i < len; i++) 
            c.d[i+j] += d[i] * b.d[j];
        for(i = 0; i < c.len-1; i++)
            c.d[i+1] += c.d[i]/10, c.d[i] %= 10;
        c.clean();
        return c;
    }
    bign operator / (const bign& b){
        int i, j;
        bign c = *this, a = 0;
        for (i = len - 1; i >= 0; i--)
        {
            a = a*10 + d[i];
            for (j = 0; j < 10; j++) if (a < b*(j+1)) break;
            c.d[i] = j;
            a = a - b*j;
        }
        c.clean();
        return c;
    }
    bign operator % (const bign& b){
        int i, j;
        bign a = 0;
        for (i = len - 1; i >= 0; i--)
        {
            a = a*10 + d[i];
            for (j = 0; j < 10; j++) if (a < b*(j+1)) break;
            a = a - b*j;
        }
        return a;
    }
    bign operator += (const bign& b){
        *this = *this + b;
        return *this;
    }

    bool operator <(const bign& b) const{
        if(len != b.len) return len < b.len;
        for(int i = len-1; i >= 0; i--)
            if(d[i] != b.d[i]) return d[i] < b.d[i];
        return false;
    }
    bool operator >(const bign& b) const{return b < *this;}
    bool operator<=(const bign& b) const{return !(b < *this);}
    bool operator>=(const bign& b) const{return !(*this < b);}
    bool operator!=(const bign& b) const{return b < *this || *this < b;}
    bool operator==(const bign& b) const{return !(b < *this) && !(b > *this);}

    string str() const{
        char s[maxn]={};
        for(int i = 0; i < len; i++) s[len-1-i] = d[i]+'0';
        return s;
    }
};

istream& operator >> (istream& in, bign& x)
{
    string s;
    in >> s;
    x = s.c_str();
    return in;
}

ostream& operator << (ostream& out, const bign& x)
{
    out << x.str();
    return out;
}