大数加减运算

#include<iostream>
#include<string>

using namespace std;

void BDplus(string str1, string str2)
{
    int len1 = str1.length();
    int len2 = str2.length();
    char *value = NULL;
    if (len1 >= len2)
    {
        //value= (char*)malloc(len1*sizeof(char));
        value = new char[len1*sizeof(char)+1];
    }
    else if (len1 <= len2)
    {
        //value = (char*)malloc(len2*sizeof(char));
        value = new char[len2*sizeof(char)+1];
    }
    int flag = 0;

    int i = 0;
    while (len1 > 0 && len2 > 0)
    {
        if (len1 <= 0 && len2 > 0)
        {
            len1 = 1;
            str1[0] = '0';
        }
        else if (len1 > 0 && len2 <= 0)
        {
            len2 = 1;
            str2[0] = '0';
        }
        int temp;
        temp = (int)(str1[len1 - 1] - '0') + (int)(str2[len2 - 1] - '0'); 
        value[i++] = (char)(temp % 10 + flag + (int)'0');
        if (temp / 10 == 1)
        {
            flag = 1;
        }
        else
        {
            flag = 0;
        }
        len1--;
        len2--;
    }
    if (flag == 1)
    {
        value[i++] = '1';
        value[i] = '';
    }
    else
    {
        value[i] = '';
    }
    i--;
    for(; i >= 0; i--)
    {
        cout << value[i];
    }
}
void BDminus(string str1, string str2)
{
    int len1, len2;
    len1 = str1.length();
    len2 = str2.length();
    int flag = 0, i = 0;
    int Case = 0;
    char *value = NULL;

    /*因为 string类重载了与string相比较的所有函数，包括<,>,==,>=,<=,!=*/

    //if (len1 == len2)//两字符串长度相等
    //{
    //    int j = 0;
    //    while ((j < len1) && (str1[j] == str2[j]))
    //    {
    //        j++;
    //    }
    //    if (j >= len1)//字符串1等于字符串2
    //    {
    //        Case = 1;
    //    }
    //    else        //字符串1不等于字符串2
    //    {
    //        if (str1[j]>str2[j])//字符串1大于字符串2
    //        {
    //            Case = 1;
    //        }
    //        else                //字符串1小于字符串2
    //        {
    //            Case = 2;
    //        }
    //    }

    //}
    //else if (len1 > len2)//字符串1长度大于字符串2
    //{
    //    Case = 1;
    //}
    //else                //字符串1长度小于字符串2
    //{
    //    Case = 2;
    //}

    if(str1>=str2)// (Case==1)//情况一 大减小
    {
        value = (char *)malloc(len1*sizeof(char));
        int temp;
        while (len1 > 0 && len2 > 0)
        {
            if (flag == 1)//借位处理
            {
                if (str1[len1 - 1] == '0')
                {
                    str1[len1 - 1] = '9';
                    flag = 1;
                }
                else if (str1[len1 - 1] != '0')
                {
                    str1[len1 - 1] = str1[len1 - 1] - 1;
                    flag = 0;
                }
            }
            if (str1[len1 - 1] >= str2[len2 - 1])
            {
                temp = (str1[len1 - 1] - '0') - (str2[len2 - 1] - '0');
                flag = 0;
            }
            else
            {
                temp = (str1[len1 - 1] - '0') + 10 - (str2[len2 - 1] - '0');
                flag = 1;
            }
            value[i++] = (char)(temp+(int)'0');
            len1--;
            len2--;
            if (len1 > 0 && len2 <= 0)
            {
                len2 = 1;
                str2[0] = '0';
            }
        }
    }
    else if (str1<str2)//(Case==2)//情况二 小减大
    {
        value = (char *)malloc(len2*sizeof(char));
        int temp;
        while (len1 > 0 && len2 > 0)
        {
            if (flag == 1)//借位处理
            {
                if (str2[len2 - 1] == '0')
                {
                    str2[len2 - 1] = '9';
                    flag = 1;
                }
                else if (str2[len2 - 1] != '0')
                {
                    str2[len2 - 1] = str2[len2 - 1] - 1;
                    flag = 0;
                }
            }
            if (str2[len2 - 1] >= str1[len1 - 1])
            {
                temp = str2[len2 - 1] -str1[len1 - 1];
                flag = 0;
            }
            else
            {
                temp = str2[len2 - 1] + 10 - str1[len1 - 1];
                flag = 1;
            }
            value[i++] = (char)(temp + (int)'0');
            len1--;
            len2--;
            if (len1 <= 0 && len2 > 0)
            {
                len1 = 1;
                str1[0] = '0';
            }
        }
        cout << "-";
    }

    value[i] = '';
    i--;
    while (i >= 0 && value[i] == '0')
    {
        i--;
    }
    if (i < 0)
    {
        cout << 0;
    }
    else
    {
        for(; i >= 0; i--)
        {
            cout << value[i];
        }
    }
}
int main(void)
{
    string str1,str2,result;
    cin >> str1>>str2;
    BDplus(str1, str2);
    BDminus(str1, str2);

    return 0;
}