Multiply Strings

 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         string product( num1.length()+num2.length(), ‘0‘ );
 7         for( size_t i = 0; i != num1.length(); ++i ) {
 8             int carry = 0;
 9             for( size_t j = 0; j != num2.length(); ++j ) {
10                 carry += product[i+j]-‘0‘ + (num1[i]-‘0‘) * (num2[j]-‘0‘);
11                 product[i+j] = carry%10 + ‘0‘;
12                 carry /= 10;
13             }
14             int k = i + num2.length();
15             while( carry ) {
16                 carry = product[k]-‘0‘ + carry;
17                 product[k++] = carry%10 + ‘0‘;
18                 carry /= 10;
19             }
20         }
21         reverse( product.begin(), product.end() );
22         string::size_type pos = product.find_first_not_of( ‘0‘ );
23         return pos == string::npos ? "0" : product.substr(pos);
24     }
25 };