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.

 1 class Solution {
 2 public:
 3     string multiply(string num1, string num2) {
 4         reverse(num1.begin(), num1.end());
 5         reverse(num2.begin(), num2.end());
 6         int nn[250] = { 0 }, maxl = 0;
 7         int l1 = num1.length(), l2 = num2.length();
 8         for(int i=0;i<l1;i++)
 9             for (int j = 0; j < l2; j++) {
10                 nn[i + j] += (num1[i] - '0')*(num2[j] - '0');
11                 int tmp = i + j;
12                 while (nn[tmp] > 9) {
13                     nn[tmp + 1] += nn[tmp] / 10;
14                     nn[tmp] %= 10;
15                     tmp++;
16                 }
17                 if (nn[tmp] == 0)continue;
18                 maxl = max(tmp, maxl);
19             }
20         string ans = "";
21         for (int i = maxl; i >= 0; i--) {
22             char ch = nn[i] + '0';
23             ans += ch;
24         }
25         return ans;
26     }
27 };

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