题解 | 大数乘法

class Solution {
public:
    string add(string a, string b) {
        reverse(a.begin(), a.end());
        reverse(b.begin(), b.end());
        int pre = 0, i = 0;
        if (a.size() > b.size()) swap(a, b);

        for (; i < a.size(); i ++) {
            int num1 = a[i] - 48, num2 = b[i] - 48;
            int sum = pre + num1 + num2;
            pre = sum / 10;
            b[i] = sum % 10 + 48;
        }

        for (; i < b.size(); i ++) {
            int num = b[i] - 48;
            int sum = pre + num;
            pre = sum / 10;
            b[i] = sum % 10 + 48;
        }

        if (pre) b.push_back('1');
        reverse(b.begin(), b.end());
        return b;
    }

    string solve(string s, string t) {
        if (s.size() < t.size()) swap(s, t);

        string ans = "";
        for (int i = 0; i < t.size(); i ++) {
            for (int j = 0; j < t[i] - 48; j ++) {
                ans = add(ans, s);
            }
            if (i != t.size() - 1) ans.push_back('0');
        }

        return ans == "" ? "0": ans;
    }
};