描述

給出兩個正整數以及四則運算操作符（+ - * /），求運算結果。

輸入第一行：正整數a，長度不超過100
第二行：四則運算子o，o是“+”，“-”，“*”，“/”中的某一個
第三行：正整數b，長度不超過100

保證輸入不含多餘的空格或其它字元輸出一行：表示式“a o b”的值。

補充說明：
1. 減法結果有可能為負數
2. 除法結果向下取整
3. 輸出符合日常書寫習慣，不能有多餘的0、空格或其它字元樣例輸入

9876543210
+
9876543210

樣例輸出

19753086420

Code:

#include<iostream>
#include<cstring>
using namespace std;

const int MAXLEN = 201;

int Substract(int *p1, int *p2, int len1, int len2) {
    int i;
    if (len1 == len2) {
        for (i = len1 - 1; i >= 0; ++i) {
            if (p1[i] < p2[i]) return -1;   //  p1 < p2
            else if (p1[i] > p2[i]) break;  //  p1 > p2
        }
    }
    for (i = 0; i < len1; ++i) {            //  要求呼叫本函式確保當i >= len2 時， p2[i] = 0
        p1[i] -= p2[i];
        if (p1[i] < 0) {
            p1[i] += 10;
            --p1[i+1];
        }
    }
    for (i = len1 - 1; i >= 0; --i) {
        if (p1[i]) return i + 1;            //  找到最高位第一個不為0
        return 0;                           //  全部為0說明兩者相等
    }
}

class Integer {
private:
    int is_neg;
    int len;
    int s[MAXLEN];
    char str[MAXLEN];

public:
    Integer(const char *string = "") {
        memset(s, 0, MAXLEN*sizeof(int));
        memset(str, 0, MAXLEN*sizeof(char));
        strcpy(str, string);
        is_neg = 0;
        len = strlen(str);
        for (int i = 0; i < len; ++i) {
            s[i] = int(str[len-1-i]) - 48;  //  s[i]低位在左， 高位在右。
        }
    }

    Integer& operator = (const Integer& oth) {
        if (this == &oth) return *this;
        memset(s, 0, MAXLEN*sizeof(int));
        memset(str, 0, MAXLEN*sizeof(char));
        is_neg = oth.is_neg;
        len = oth.len;
        for (int i = 0; i < oth.len; ++i) {
            s[i] = oth.s[i];
        }
        strcpy(str, oth.str);
        return *this;
    }

    bool operator == (const Integer& oth) {
        if (this == &oth) return true;
        bool ret = true;
        if (len != oth.len || is_neg != oth.is_neg)
            ret = false;
        if (strcmp(str, oth.str)) ret = false;
        for (int i = 0; i < oth.len; ++i) {
            if (s[i] != oth.s[i]) ret = false;
        }
        return ret;
    }

    bool operator != (const Integer& oth) {
        return !(*this == oth);
    }

    Integer operator+(const Integer& oth) {
        Integer c;
        int length = len >= oth.len ? len : oth.len;
        length += 1;
        c.len = length;
        for (int i = 0; i < c.len; ++i) {
            c.s[i] += s[i] + oth.s[i];
            c.s[i+1] += c.s[i] / 10;
            c.s[i] = c.s[i] % 10;
        }

        while ((c.len > 1) && (c.s[c.len-1] == 0))
            c.len--;

        return c;
    }

    Integer operator-(const Integer& oth) {
        Integer c;
        int flag = 0;

        if (len > oth.len) flag = 1;
        else if (len < oth.len) flag = -1;
        else flag = strcmp(str, oth.str);

        if (flag >= 0) {
            c.len = len;
            int borrow = 0;
            for (int i = 0; i < c.len; ++i) {
                c.s[i] += s[i] - oth.s[i];
                if (borrow) c.s[i] -= 1;
                if (c.s[i] < 0) {
                    c.s[i] += 10;
                    borrow = 1;
                } else {
                    borrow = 0;
                }
            }
            while ((c.len > 1) && (c.s[c.len-1] == 0))
                c.len--;
        } else {
            c.is_neg = 1;
            c.len = oth.len;
            int borrow = 0;
            for (int i = 0; i < c.len; ++i) {
                c.s[i] += oth.s[i] - s[i];
                if (borrow) c.s[i] -= 1;
                if (c.s[i] < 0) {
                    c.s[i] += 10;
                    borrow = 1;
                } else {
                    borrow = 0;
                }
            }
            while ((c.len > 1) && (c.s[len-1] == 0))
                c.len--;
        }
        return c;
    }

    Integer operator*(const Integer& oth) {
        Integer c;
        c.len = len + oth.len + 1;
        for (int i = 0; i < len; ++i) {
            for (int j = 0; j < oth.len; ++j) {
                c.s[i+j] += s[i] * oth.s[j];
                c.s[i+j+1] += c.s[i+j] / 10;
                c.s[i+j] = c.s[i+j] % 10;
            }
        }
        while ((c.len > 1) && (c.s[c.len-1] == 0))
            c.len--;
        
        return c;
    }

    Integer operator/(Integer& oth) {
        Integer c;
        c.len = MAXLEN;
        int i, temp;
        if (len < oth.len) {
            for (int i = 0; i < MAXLEN; ++i) {
                c.s[i] = 0;
            }
            return c;
        }
        int nTimes = len - oth.len;
        if (nTimes > 0) {
            for (i = len-1; i >= nTimes; --i) {
                oth.s[i] = oth.s[i-nTimes];
            }
            for (; i >= 0; --i) {
                oth.s[i] = 0;
            }
            oth.len = len;
        }
        for (i = 0; i <= nTimes; ++i) {
            while ((temp = Substract(s, oth.s+i, len, oth.len-i)) >= 0) {
                len = temp;
                ++c.s[nTimes-i];
            }
        }
        while ((c.len > 1) && (c.s[c.len-1] == 0))
            c.len--;
        
        return c;
    }

    friend ostream & operator<<(ostream& out, const Integer& oth) {
        if (oth.is_neg) out << "-";
        for (int k = oth.len-1; k >= 0; --k) 
            out << oth.s[k];
        return out;
    }
};

int main() {
    char s1[MAXLEN], s2[MAXLEN];
    char ope;
    cin >> s1;
    cin >> ope;
    cin >> s2;
    Integer a(s1);
    Integer b(s2);

    switch (ope) {
        case '+': 
            cout << a + b << endl;
            break;
        case '-':
            cout << a - b << endl;
            break;
        case '*':
            cout << a * b << endl;
            break;
        case '/':
            cout << a / b << endl;
            break;
    }

    return 0;
}

reference:

https://blog.csdn.net/ApplePeel_90/article/details/80066215