80分求助！

#include <bits/stdc++.h>
using namespace std;
string num1, num2;
int a1[2200], a2[2200], len1, len2, a[2200], len;
int main() {
    cin >> num1 >> num2;
    len1 = num1.size();
    if (num1 == "0" || num2 == "0") {
        cout << 0 ;
        return 0;
    }
    for (int i = 0; i < len1; i++) {
        a1[i] = num1[len1 - i - 1] - '0';
    }
    len2 = num2.size();
    for (int i = 0; i < len2; i++) {
        a2[i] = num2[len2 - 1 - i] - '0';
    }
    for (int i = 0; i < len1; i++) {
        for (int j = 0; j < len2; j++) {
            a[i + j] += a1[i] * a2[j];
        }
    }
    len = len1 + len2 - 1;
    for (int i = 0; i < len; i++) {
        a[i + 1] += a[i] / 10;
        a[i] %= 10;
    }
    while (a[len]) {
        a[len + 1] += a[len] / 10;
        a[len] %= 10;
        len++;
    }
    for (int i = len - 1; i >= 0; i--) {
        cout << a[i];
    }
    return 0;
}