大整数类

大整数类主要就是运算符的重载问题，除<<和>>外其他的符号都要写在struct的外面，而且只要定义了一个<符号之后，其它的符号都可以根据<的关系推导出来

下面贴上代码

注意给字符串赋值的时候，如果声明就开始赋值，那么必须得定义一个函数BigInteger(string/char[]/long long) 

#include<vector>
#include<iostream>
#include<string>
#include<cstdio>
using namespace std;
struct BigInteger
{
    static const int WIDTH = 8;
    static const int BASE = 100000000;//千万级别
    vector<int>s;
    BigInteger(long long num = 0){*this = num;}//构造函数，其中的=号，需要等会重载一下
    BigInteger(string str){*this = str;}
    BigInteger operator = (long long num)
    {
        s.clear();
        do
        {
            s.push_back(num % BASE);
            num = num / BASE;
        }while(num);
        return *this;
    }
    BigInteger operator = (string str)
    {
        s.clear();
        int start = str.length(),last = str.length();
        do
        {
            int x;
            start = max(start - WIDTH,0);
            sscanf(str.substr(start,last-start).c_str(),"%d",&x);//特别注意：substr的第二个参数是从start开始的有多少个字符
            s.push_back(x);
            last = start;
        }while(start);
        return *this;
    }
    BigInteger operator + (BigInteger B)
    {
        BigInteger c;
        c.s.clear();
        int g = 0;
        for(int i=0,j=0;j<B.s.size()||i<this->s.size()||g;i++,j++)
        {
            if(i<s.size())
            {
                g=g+s[i];
            }
            if(j<B.s.size())
            {
                g=g+B.s[j];
            }
            c.s.push_back(g%BASE);
            g=g/BASE;
        }
        return c;
    }
    void output()
    {
        for(int i=s.size()-1;i>=0;i--)
        {
            cout<<s[i];
        }
    }
    bool operator < (BigInteger B)
    {
        if(s.size() < B.s.size())
            return true;
        if(s.size() > B.s.size())
            return false;
        for(int i = s.size() - 1;i >= 0;i--)
        {
            if(s[i] < B.s[i])
                return true;
            if(s[i] > B.s[i])
                return false;
        }
        return false;//相等
    }
    bool operator > (BigInteger B)
    {
        if(B < *this)
            return true;
        return false;
    }
    bool operator <= (BigInteger B)
    {
        if(*this > B)
        {
            return false;
        }
        return true;
    }
    bool operator >= (BigInteger B)
    {
        if(*this < B)
            return false;
        return true;
    }
    bool operator == (BigInteger B)
    {
        if(*this > B)
            return false;
        if(*this < B)
            return false;
        return true;
    }
    bool operator != (BigInteger B)
    {
        if(*this == B)
        {
            return false;
        }
        else
        {
            return true;
        }
    }
};

ostream& operator << (ostream &out,  BigInteger B)
    {
        for(int i=B.s.size()-1;i>=0;i--)
        {
            cout<<B.s[i];
        }
        return out;
    }
istream& operator >> (istream &in, BigInteger B)
{
    string s;
    if(!(in>>s))return in;
    B=s;
    return in;
}
int main()
{
    vector<BigInteger>V;
    BigInteger x("1234567890123456789012345678901234567890");
    BigInteger y("9876543210987654321");
    BigInteger z("3426245");
    BigInteger G("3426245");
    V.push_back(x);
    V.push_back(y);
    V.push_back(z);
    if(x>=y)printf(">=\n");
    if(y<=y)printf("<=\n");
    if(z==G)printf("<=\n");
    if(x!=y)printf("!=\n");
    x.output();
    return 0;
}