Leetcode#43 Multiply Strings

 1 string multiply(string num1, string num2) {
 2         int len1 = num1.length();
 3         int len2 = num2.length();
 4         vector<int> buffer(len1 + len2, 0);
 5         
 6         for (int i = len1 - 1; i >= 0; i--)
 7             for (int j = len2 - 1; j >= 0; j--)
 8                 buffer[len1 - 1 - i + len2 - 1 - j] += (num1[i] - ‘0‘) * (num2[j] - ‘0‘);
 9         
10         for (int i = 0, carry = 0; i < len1 + len2; i++) {
11             buffer[i] += carry;
12             carry = buffer[i] / 10;
13             buffer[i] %= 10;
14         }
15         
16         string res;
17         int p = len1 + len2 - 1;
18         while (p >= 0 && buffer[p] == 0) p--;
19         while (p >= 0) {
20             res += buffer[p] + ‘0‘;
21             p--;
22         }
23         
24         return res.empty() ? "0" : res;
25     }