大数乘法

#include <bits/stdc++.h>

using namespace std;

vector<int> multiplication(const vector<int> &a, const vector<int> &b) {
    long la = a.size();
    long lb = b.size();
    vector<int> res(la + lb - 1);
    for (int i = 0; i < la; i++) {
        for (int j = 0; j < lb; j++) {
            res[i + j] += a[i] * b[j];
        }
    }
    int carry = 0;
    for (int i = la + lb - 2; i >= 0; i--) {
        res[i] += carry;
        carry = res[i] / 10;
        res[i] %= 10;
    }
    if (carry) {
        res.insert(res.begin(), carry);
    }
    return res;
}

int main() {
    vector<int> a = {9, 9, 9, 9, 9}, b = {9, 9, 9, 9, 9};
    auto res = multiplication(a, b);
    for (auto e:res)
        cout << e;
    cout << endl;
    return 0;
}

输出：
9999800001