使用string实现一个用于储存那些太大而无法使用 long long 的数

类的定义：

class stringInt {
public:
    stringInt();
    stringInt(string num);
    stringInt(int num);
    stringInt operator+(stringInt & num);
    stringInt operator-(stringInt & num);
        
    // ... 还可以乘除之类的操作

private:
    string str;
};

所用的函数：

// 将string和int相互转换
string num2string(int num);
int string2num(string str);
int compareTwoNum(string firstStr, string secondStr);  // 比较两个用string表示的数的大小
int twoCharAdd(char ch1, char ch2, bool moreOne);  // 对两个用char表示的数的加操作
int twoCharSub(char ch1, char ch2, bool &lessOne);  // 对两个用char表示的数的减操作
void transpose(string &str);  // 将一个string转置

在对加、减操作的实现中，主要依赖stringAdd()、stringSubtract()函数，用于对两个用string表示的数做加减

具体实现：

string stringAdd(string firstStr, string secondStr)
{
    string outStr;
    int firstStrLen, secondStrLen, sum;
    firstStrLen = firstStr.length();
    secondStrLen = secondStr.length();
    bool moreOne = false;
    string temp = "0";

    //cout << firstStr << endl;
    //cout << secondStr << endl;
    //cout << firstStrLen << "  " << secondStrLen << endl;

    //cout << "Init outStr: " << outStr << endl;

    if (firstStrLen >= secondStrLen) {
        //int cnt = firstStrLen;

        for (int i = firstStrLen - 1; i >= 0; i--) {

            //cout << "i = " << i << endl;

            if (secondStrLen > 0) {
                sum = twoCharAdd(firstStr[i], secondStr[secondStrLen - 1], moreOne);

                cout << "sum = " << sum << endl;

                if (sum >= 10) {
                    moreOne = true;
                    outStr.append(num2string(sum - 10));   
                    // string 没有在字符串头部增加一个字符的函数，虽然实现这个函数并不复杂，但此处如果使用头部
                    // 增加一个元素的函数，则在每一个循环都需要使用，与使用append() 再 将结果字符串反转相比，需要
                    // 的代价相差太大，所以此处选用后一种方法
                }
                else {
                    moreOne = false;
                    outStr.append(num2string(sum));
                }

                secondStrLen--;

                //cout << "secondStrLen = " << secondStrLen << "  " << outStr << endl;

                continue;
            }

            if (moreOne) {
                sum = twoCharAdd(firstStr[i], '0', moreOne);

                //cout << "No continue sum = " << sum << endl;

                if (sum >= 10) {
                    moreOne = true;
                    outStr.append(num2string(sum - 10));
                }
                else {
                    moreOne = false;
                    outStr.append(num2string(sum));
                }
            }
            else {
                if (i != 0) {    // 当 i = 0 时，&firstStr[0] 表示整个 firstStr 字符串
                    outStr.append(&firstStr[i]);  
                }
                else {
                    temp[0] = firstStr[i];
                    outStr.append(temp);
                }
                

            }
        }
        //cout << "End for outStr = " << endl;
    }
    else {
        int cnt = secondStrLen;
        for (int i = cnt - 1; i >= 0; i--) {
            if (firstStrLen > 0) {
                sum = twoCharAdd(firstStr[firstStrLen - 1], secondStr[i], moreOne);

                if (sum >= 10) {
                    moreOne = true;
                    outStr.append(num2string(sum - 10));
                }
                else {
                    moreOne = false;
                    outStr.append(num2string(sum));
                }

                firstStrLen--;
                continue;
            }

            if (moreOne) {
                sum = twoCharAdd('0', secondStr[i], moreOne);

                if (sum >= 10) {
                    moreOne = true;
                    outStr.append(num2string(sum - 10));
                }
                else {
                    moreOne = false;
                    outStr.append(num2string(sum));
                }
            }
            else {
                if (i != 0) {   
                    outStr.append(&secondStr[i]);
                }
                else {
                    temp[0] = secondStr[i];
                    outStr.append(temp);
                }
            }
        }
    }

    transpose(outStr);

    return outStr;
}

string stringSubtract(string firstStr, string secondStr)  // 第一个是减数，第二个是被减数
{
    int firstStrLen, secondStrLen;
    string outStr;
    firstStrLen = firstStr.length();
    secondStrLen = secondStr.length();

    //cout << "初始outStr：" << outStr << endl;

    //cout << "两字符串长度为：" << firstStrLen << "  " << secondStrLen << endl;
    //cout << "两字符串为：" << firstStr << "  " << secondStr << endl;

    char ch = '0';
    string minus = "-";
    bool lessOne = false;

    //cout << "compareTN: " << compareTwoNum(firstStr, secondStr) << endl;

    if (compareTwoNum(firstStr, secondStr) == 1) {
        for (int i = firstStrLen - 1; i >= 0; i--) {
            if (secondStrLen > 0) {
                int sub = twoCharSub(firstStr[i], secondStr[secondStrLen -1], lessOne);
                
                cout << "sub = " << sub << endl;
                cout << "n2s_sub = " << num2string(sub) << endl;
                
                outStr.append(num2string(sub));
                secondStrLen--;

                //cout << outStr << endl;

                continue;
            }

            int sub = twoCharSub(firstStr[i], '0', lessOne);
            outStr.append(num2string(sub));

            //cout << outStr << endl;
        }

        //cout << outStr << endl;
    }
    else if (compareTwoNum(firstStr, secondStr) == -1) {
        for (int i = secondStrLen - 1; i >= 0; i--) {
            if (firstStrLen > 0) {
                int sub = twoCharSub(secondStr[i], firstStr[firstStrLen - 1], lessOne);
                outStr.append(num2string(sub));
                firstStrLen--;
                continue;
            }

            int sub = twoCharSub(secondStr[i], '0', lessOne);
            outStr.append(num2string(sub));
        }

        //cout << "加负号前 outStr = " << outStr << endl;

        outStr.append(minus);
    }
    else {
        outStr.append(&ch);
    }

    //cout << "加负号后，转置前 outStr = " << outStr << endl;

    transpose(outStr);

    return outStr;
}

string stringMultiply(string firstStr, string secondStr)
{
    return string();
}

将string和int相互转换

string num2string(int num)
{
    //cout << "num = " << num << endl;

    stringstream ss;
    ss << num;

    string str = ss.str();

    //cout << str << endl;

    return str;
}

int string2num(string str)
{
    stringstream ss;
    ss << str;
    int num;
    ss >> num;

    return num;
}

其他的辅助函数的实现

int twoCharAdd(char ch1, char ch2, bool moreOne)
{
    int sum, num1, num2;
    num1 = string2num(&ch1);
    num2 = string2num(&ch2);

    if (moreOne) {
        sum = num1 + num2 + 1;
    }
    else {
        sum = num1 + num2;
    }

    return sum;
}

void transpose(string &str)
{
    int len = str.length();

    for (int i = 0; i < len / 2; i++) {
        char tempChar = str[i];
        str[i] = str[len - 1 - i];
        str[len - 1 - i] = tempChar;
    }

    return;
}

int compareTwoNum(string firstStr, string secondStr)
{
    int firstStrLen, secondStrLen;
    firstStrLen = firstStr.length();
    secondStrLen = secondStr.length();

    if (firstStrLen > secondStrLen) {
        return 1;
    }
    else if (firstStrLen < secondStrLen) {
        return -1;
    }
    else {
        for (int i = 0; i < firstStrLen; i++) {
            if (string2num(&firstStr[i]) > string2num(&secondStr[i])) {
                return 1;
            }
            else if (string2num(&firstStr[i]) < string2num(&secondStr[i])){
                return -1;
            }
            else {
                continue;
            }
        }
    }

    return 0;
}

int twoCharSub(char ch1, char ch2, bool &lessOne)
{
    //cout << ch1 << "  " << ch2 << "  " << lessOne << endl;

    int num1, num2;

    if (lessOne) {
        num1 = string2num(&ch1) - 1;
    }
    else {
        num1 = string2num(&ch1);
    }
    
    num2 = string2num(&ch2);

    if (num1 >= num2) {
        lessOne = false;
        return num1 - num2;
    }
    else {
        lessOne = true;
        return 10 + num1 - num2;
    }
}

收获：

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