19.2.4 [LeetCode 43] Multiply Strings

Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string.

Example 1:

Input: num1 = "2", num2 = "3"
Output: "6"

Example 2:

Input: num1 = "123", num2 = "456"
Output: "56088"

Note:

1. The length of both num1 and num2 is < 110.
2. Both num1 and num2 contain only digits 0-9.
3. Both num1 and num2 do not contain any leading zero, except the number 0 itself.
4. You must not use any built-in BigInteger library or convert the inputs to integer directly.

class Solution {
public:
    string multiply(string num1, string num2) {
        reverse(num1.begin(), num1.end());
        reverse(num2.begin(), num2.end());
        int nn[250] = { 0 }, maxl = 0;
        int l1 = num1.length(), l2 = num2.length();
        for(int i=0;i<l1;i++)
            for (int j = 0; j < l2; j++) {
                nn[i + j] += (num1[i] - '0')*(num2[j] - '0');
                int tmp = i + j;
                while (nn[tmp] > 9) {
                    nn[tmp + 1] += nn[tmp] / 10;
                    nn[tmp] %= 10;
                    tmp++;
                }
                if (nn[tmp] == 0)continue;
                maxl = max(tmp, maxl);
            }
        string ans = "";
        for (int i = maxl; i >= 0; i--) {
            char ch = nn[i] + '0';
            ans += ch;
        }
        return ans;
    }
};

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