Multiply Strings 大数相乘

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.

class Solution {
public:
    string multiply(string num1,string num2) {
        if(num1=="0"||num2=="0")
            return "0";
        int i,j,len1,len2;
        len1=num1.size();
        len2=num2.size();
        int *res=new int[len1+len2];
        memset(res,sizeof(int)*(len1+len2));
        for(i=0;i<len1;i++)
        {
            for(j=0;j<len2;j++)
            {
                res[i+j+1]+=(num1[i]-'0')*(num2[j]-'0');//res[0]空出来最高位进位
            }
        }
        
        string str="";
        for(i=len1+len2-1;i>=0;i--)
        {
            if(res[i]>=10)
                res[i-1]+=res[i]/10;
            res[i]%=10;
            str=char(res[i]+'0')+str;
        }
        if(str[0]=='0')
            str=str.substr(1);
        return str;
    }
};