大整数运算

int最大是2147483647，long long最大是9223372036854775807
故而超过18位的数需要用字符串来运算。

两个大整数相加

#include<iostream>
#include<string>
using namespace std;
int main()
{
       string s1, s2, sum="";
       cin >> s1 >> s2;
       int n1 = s1.size()-1, n2 = s2.size()-1, a = 0;
       while (n1 >= 0 || n2 >= 0 || a == 1)
       {
              if (n1 >= 0)
              {
                     a += s1[n1] - '0';
                     n1--;
              }
              if (n2 >= 0)
              {
                     a += s2[n2] - '0';
                     n2--;
              }
              sum = to_string(a % 10) + sum;
              a /= 10;
       }
       cout << sum << endl;
       system("pause");
       return 0;
}

 

两个大整数相减

 

#include<iostream>
#include<string>
using namespace std;
int main()
{
       string s1, s2, res="";
       cin >> s1 >> s2;
       int n1 = s1.size() - 1, n2 = s2.size() - 1, a = 0;
       if (n1 < n2)  //s1为较大的数
       {
              string t = s2;
              s2 = s1;
              s1 = t;
       }
       int len = n1 - n2;  //s2补全0
       string s3(len, '0');
       s2 = s3 + s2;
       for (int i = n1; i >= 0; i--)
       {
              a = (s1[i] - '0') - (s2[i] - '0') - a;
              if (a < 0)
              {
                     res = to_string(a + 10) + res;
                     a = 1;
              }
              else
              {
                     res = to_string(a) + res;
                     a = 0;
              }
       }
       if (res[0] == '0')
       {
              cout << res.substr(1) << endl;
       }
       else
       {
              cout << res << endl;
       }
       system("pause");
       return 0;
}

 

两个大整数相乘

 

#include<iostream>
#include<string>
using namespace std;
string BigNumMultiply(string str1, string str2)
{
       int size1 = str1.size(), size2 = str2.size();
       string str(size1 + size2, '0');  //str保存乘法结果
       for (int i = size2 - 1; i >= 0; --i)
       {
              int mulflag = 0, addflag = 0;  //mulflag是乘法的进位，addflag是加法的进位
              for (int j = size1 - 1; j >= 0; --j)
              {
                     int temp1 = (str2[i] - '0')*(str1[j] - '0') + mulflag;  //逐位相乘
                     mulflag = temp1 / 10;
                     temp1 = temp1 % 10;
                     int temp2 = str[i + j + 1] - '0' + temp1 + addflag;  //第i+j+1位加上相乘结果
                     str[i + j + 1] = temp2 % 10 + '0';
                     addflag = temp2 / 10;
              }
              str[i] += mulflag + addflag;  //第i位加上进位
       }
       if (str[0] == '0')  //首位是0则去掉
              str = str.substr(1);
       return str;
}
int main()
{
       string str1, str2;
       cin >> str1 >> str2;
       cout << BigNumMultiply(str1, str2) << endl;
       system("pause");
       return 0;
}

 

两个大整数相除

 

#include<iostream>
#include<string>
using namespace std;
string BigNumMultiply(string str1, string str2)
{
       int size1 = str1.size(), size2 = str2.size();
       string str(size1 + size2, '0');
       for (int i = size2 - 1; i >= 0; --i)
              {
              int mulflag = 0, addflag = 0;
              for (int j = size1 - 1; j >= 0; --j)
                     {
                     int temp1 = (str2[i] - '0')*(str1[j] - '0') + mulflag;
                     mulflag = temp1 / 10;
                     temp1 = temp1 % 10;
                     int temp2 = str[i + j + 1] - '0' + temp1 + addflag;
                     str[i + j + 1] = temp2 % 10 + 48;
                     addflag = temp2 / 10;
                     }
              str[i] += mulflag + addflag;
              }
       if (str[0] == '0')
              str = str.substr(1, str.size());
       return str;
       }
int main()
{
       string str1, str2;
       while (cin >> str1 >> str2)
       {
              cout << str1 << "*" << str2 << "=" << endl;
              cout << BigNumMultiply(str1, str2) << endl;
       }
       system("pause");
       return 0;
}