bignum 大数模板

　　今天无意间看到一个很好的大数模板，能算加、减、乘、除等基本运算，但操作减法的时候只能大数减小数，也不支持负数，如果是两个负数的话去掉符号相加之后再取反就可以了，一正一负比较绝对值大小，然后相减。我借用了一下：（作过少许代码上的精简）

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

const int maxn = 2003;

struct bign{
    int len, s[maxn];
    bign(){
        memset(s,0,sizeof(s));
        len= 1;
    }
    bign(int num)    {    *this = num;    }
    bign(const char *num)    {    *this = num;    }
    bign operator = (const int num){
        char s[maxn];
        sprintf(s,"%d",num);
        return *this = s;
    }
    bign operator = (const char *num){
        for(int i = 0; num[i] && num[i] =='0'; ++num) ;
        len = strlen(num);
        for(int i = 0; i < len; ++i)
            s[i] = num[len-1-i] -'0';
        return *this;
    }
    bign operator + (const bign &b) const {
        bign c;
        c.len = 0;
        int maxlen = max(len, b.len);
        for(int i = 0, g = 0; g || i < maxlen; ++i){    // g是进位 
            int x = g;
            if(i < len)      x += s[i];
            if(i < b.len)    x += b.s[i];
            c.s[c.len++] = x % 10;
            g = x / 10;
        }
        return c;
    }
    bign operator += (const bign &b){
        return *this = *this + b;
    }
    void clean(){
        while(len >1 && !s[len-1])    --len;
    }
    bign operator * (const bign &b) const {
        bign c;
        c.len = len + b.len;
        
        for(int i = 0; i < len; ++i)
            for(int j = 0; j < b.len; ++j)
                c.s[i+j] += s[i] * b.s[j];
                
        for(int i = 0; i < c.len; ++i){
            c.s[i+1] += c.s[i]/10;
            c.s[i] %= 10;
        }
        c.clean();
        return c;
    }
    bign operator *= (const bign &b){
        return *this = *this * b;
    }
    bign operator -(const bign &b) const {
        bign c;
        c.len = 0;
        for(int i = 0, g = 0; i < len; ++i){    //此时的 g是退位
            int x = s[i]- g;
            if(i < b.len)    x -= b.s[i];
            if(x >= 0)    g = 0;
            else {
                g = 1;
                x += 10;
            }
            c.s[c.len++] = x;
        }
        c.clean();
        return c;
    }
    bign operator -= (const bign &b){
        return *this = *this - b;
    }
    bign operator / (const bign &b) const {
        bign c, f = 0;
        for(int i = len-1; i >= 0; --i){
            f *= 10;
            f.s[0] = s[i];
            while(f >= b){
                f -= b;
                ++c.s[i];
            }
        }
        c.len = len;
        c.clean();
        return c;
    }
    bign operator /= (const bign &b){
        return *this = *this / b;
    }
    bign operator % (const bign &b) const {
        bign r = *this / b;
        return *this- r*b;
    }
    bign operator %= (const bign &b){
        return *this = *this % b;
    }
    bool operator < (const bign &b) const {
        if(len != b.len)    return len < b.len;
        for(int i= len-1; i>=0; --i)
            if(s[i] != b.s[i])    return s[i] < b.s[i];
        return 0;
    }
    bool operator > (const bign &b) const {
        if(len != b.len)    return len > b.len;
        for(int i = len-1; i >= 0; --i)
            if(s[i] != b.s[i])    return s[i] > b.s[i];
        return 0;
    }
    bool operator == (const bign &b) const {
        return !(*this < b || *this > b);
    }
    bool operator != (const bign &b) const {
        return !(*this == b);
    }
    bool operator <= (const bign &b) const {
        return *this < b || *this == b;
    }
    bool operator >= (const bign &b) const {
        return *this > b || *this == b;
    }
    string str() const {
        string res = "";
        for(int i = 0; i < len; ++i)
            res = char(s[i]+'0') + res;        //这里一定不能掉乱 +的顺序！ 
        return res;
    }
};

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;
}

int main()
{
    bign a,b,c,d,e,f,g;
    while(cin>>a>>b)
    {
        a.clean();
        b.clean();
        c = a+b;
        d = a-b;
        e = a*b;
        f = a/b;
        g = a%b;
        cout<<"a+b = "<<c<<endl;
        cout<<"a-b = "<<d<<endl;
        cout<<"a*b = "<<e<<endl;
        cout<<"a/b = "<<f<<endl;
        cout<<"a%b = "<<g<<endl;
        cout<<(a==b? "a = b":"a != b")<<endl;
    }
    return 0;
}

　　在此感谢原博客的模板提供： bign类C++高精度模板