bigInt 大整数及加法乘法模板

#include <bits/stdc++.h>
using namespace std;

struct bigInt {
    int digit[1000] = {0}, len = 0;
    // s 的默认值为 “0”，可以不用传参
    bigInt (string s = "0") {
        len = s.size();
        for (int i = 0, j = len-1; i < len; i++, j--)
            digit[i] = s[j] - '0';
    }
    // 重载构造函数，支持传整数
    bigInt (int n) {
        // 处理传进来 0 的特殊情况
        if (n == 0) len = 1;
        while (n) {
            digit[len++] = n % 10;
            n /= 10;
        }
    }
    void print() {
        for (int i = len-1; i >= 0; i--)
            cout << digit[i];
        cout << endl;
    }
};

bigInt operator+(bigInt a, bigInt b) {
    bigInt c;
    c.len = max(a.len, b.len);
    for (int i = 0; i < c.len; i++) {
        c.digit[i] += a.digit[i] + b.digit[i];
        c.digit[i+1] += c.digit[i] / 10;
        c.digit[i] %= 10;
    }
    if (c.digit[c.len]) c.len++;
    return c;
}

// 仅支持 a >= b 的减法
bigInt operator-(bigInt a, bigInt b) {
    bigInt c;
    c.len = max(a.len, b.len);
    for (int i = 0; i < c.len; i++) {
        c.digit[i] += a.digit[i] - b.digit[i];
        if (c.digit[i] < 0) {
            c.digit[i] += 10;
            c.digit[i+1]--;
        }
    }
    while (c.len > 1 && c.digit[c.len-1] == 0) c.len--;
    return c;
}

bigInt operator*(int a, bigInt b) {
    for (int i = 0; i < b.len; i++)
        b.digit[i] *= a;
    for (int i = 0; i < b.len; i++) {
        b.digit[i+1] += b.digit[i] / 10;
        b.digit[i] %= 10;
    }
    while (b.digit[b.len]) {
        b.len++;
        b.digit[b.len] += b.digit[b.len-1] / 10;
        b.digit[b.len-1] %= 10;
    }
    return b;
}

bigInt operator*(bigInt a, int b) {
    return b * a;
}

bigInt operator*(bigInt a, bigInt b) {
    bigInt c;
    c.len = a.len + b.len - 1;
    for (int i = 0; i < a.len; i++)
        for (int j = 0; j < b.len; j++)
            c.digit[i+j] += a.digit[i] * b.digit[j];
    for (int i = 0; i < c.len; i++) {
        c.digit[i+1] += c.digit[i] / 10;
        c.digit[i] %= 10;
    }    
    if (c.digit[c.len]) c.len++;
    return c;
}

int main() {
    
    string s1, s2;
    cin >> s1 >> s2;
    bigInt a = bigInt(s1) * bigInt(s2);
    cout << a.len << endl;
    a.print();

    return 0;
}