[LeetCode]Multiply Strings

 1 class Solution {
 2 public:
 3     string multiply(string num1, string num2) {
 4         int len1 = num1.size();
 5         int len2 = num2.size();
 6         
 7         int len = len1 + len2;
 8         string r(len, ‘0‘);
 9         
10         for (int i = 0; i < len1; ++i) {
11             int carry = 0;
12             int a = num1[len1 - i - 1] - ‘0‘;
13             for (int j = 0; j < len2; ++j) {
14                 int b = num2[len2 - j - 1] - ‘0‘;
15                 int sum = a * b + r[i + j] - ‘0‘ + carry;
16                 r[i + j] = sum % 10 + ‘0‘;
17                 carry = sum / 10;
18             }
19             
20             if (carry > 0) {
21                 r[i + len2] += carry;
22             }
23         }
24         
25         int start = 0;
26         for (int i = len - 1; i >= 0; --i) {
27             if (r[i] != ‘0‘) {
28                 start = i;
29                 break;
30             }
31         }
32         
33         string result;
34         for (int i = start; i >= 0; --i) {
35             result += r[i];
36         }
37         
38         return result;
39     }
40 };