leetcode Multiply Strings

题目：

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.
这道题是利用string来模拟乘法，代码如下：

class Solution {
public:
string add(string s1, string s2)
{
if (s2.empty())
{
return s1;
}
if (s1.empty())
{
return s2;
}
int carry = 0;
int m = s1.size();
int n = s2.size();
reverse(s1.begin(), s1.end());
reverse(s2.begin(), s2.end());
string result;
for (int i = 0; i < m || i < n; i++)
{
int m1 = i < m ? s1[i] - '0' : 0;
int m2 = i < n ? s2[i] - '0' : 0;
int sum = m1 + m2 + carry;
carry = sum / 10;
result.push_back(sum % 10 + '0');
}
if (carry)
{
result.push_back(carry + '0');
}
reverse(result.begin(), result.end());
return result;
}
string multiply(string num1, string num2)
{
int m = num1.size();
int n = num2.size();
reverse(num1.begin(), num1.end());
reverse(num2.begin(), num2.end());
string result;
for (int i = 0; i < m; i++)
{
string tmp;
int carry = 0;
for (int j = 0; j < n; j++)
{
int m1 = num1[i] - '0';
int m2 = num2[j] - '0';
int sum = m1 * m2 + carry;
carry = sum / 10;
tmp.push_back(sum % 10 + '0');
}
if (carry)
{
tmp.push_back(carry + '0');
}
reverse(tmp.begin(), tmp.end());
for (int k = 0; k < i; k++)
{
tmp.push_back('0');
}
result = add(result, tmp);
}
if (result[0] == '0')
{
return "0";
}
return result;
}
};