Multiply Strings leetcode

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

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ACM竞赛常考的大数相乘算法，利用字符串来表示大数字进行计算。

string multiply(string num1, string num2) {
    string num(num1.size() + num2.size(), '0');
    for (int i = num1.size() - 1; i >= 0; --i)
    {
        int carry = 0;
        for (int j = num2.size() - 1; j >= 0; --j)
        {
            int tmp = num[i + j + 1] - '0' + (num1[i] - '0') * (num2[j] - '0') + carry;
            num[i + j + 1] = tmp % 10 + '0';
            carry = tmp / 10;
        }
        num[i] += carry;
    }
    size_t startpos = num.find_first_not_of('0');
    if (startpos != string::npos)
        return num.substr(startpos);
    return "0";
}