Leetcode | Multiply Strings

 1 class Solution {
 2 public:
 3     string multiply(string num1, string num2) {

 4         if (num1.empty()) return "";
 5         if (num2.empty()) return "";
 6         int m1, m2, carry, val;
 7         
 8         reverse(num1.begin(), num1.end());
 9         reverse(num2.begin(), num2.end());
10         string ret(num1.length() + num2.length(), ‘0‘);
11         for (int i = 0; i < num2.length(); ++i) {
12             m2 = num2[i] - ‘0‘;
13             carry = 0;
14             for (int j = 0; j < num1.length(); ++j) {
15                 m1 = num1[j] - ‘0‘;
16                 val = m2 * m1 + carry + ret[i + j] - ‘0‘;
17                 ret[i + j] = val % 10 + ‘0‘;
18                 carry = val / 10;
19             }
20             ret[i + num1.length()] += carry;
21         }
22 
23         string r = "0";
24         for (int i = ret.length() - 1; i >= 0; --i) {
25             if (ret[i] != ‘0‘) {
26                 r = ret.substr(0, i + 1);
27                 break;
28             }
29         }
30         reverse(r.begin(), r.end());
31         return r;
32     }
33 };