字符串四则运算

string (string num1, string num2) {
        if(num1.size()==0)return num2;
        if(num2.size()==0)return num1;
        string res;
        int long_index=0;
        int short_index=0;
        if(num1.size()>=num2.size())
        {
            long_index=num1.size();
            short_index=num2.size();
        }
        else
        {
            long_index=num2.size();
            short_index=num1.size();
            swap(num1,num2);
        }
        int carry=0;
        while(short_index>0)
        {
            int temp=num1[--long_index]-'0'+num2[--short_index]-'0'+carry;
            res+='0'+(temp)%10;
            carry=temp/10;           
        }
        while(long_index>0)
        {
            int temp=num1[--long_index]-'0'+carry;
            res+=temp%10+'0';
            carry=temp/10;
        }
        if(carry==1)res+=1+'0';
        reverse(res.begin(),res.end());
        return res;
    }

string multiply(string num1, string num2) {
        if(num1.size()==0||num2.size()==0)return "";
        if(num1[0]=='0'||num2[0]=='0')return "0";
        string res;
        int l1=num1.size();
        int l2=num2.size();
        int num[l1+l2]{0};
        for(int i=l1-1;i>=0;i--)
            for(int j=l2-1;j>=0;j--)
                num[i+j+1]+=(num1[i]-'0')*(num2[j]-'0');
        int carry=0;
        for(int i=l1+l2-1;i>=0;i--)
        {
            num[i]+=carry;
            carry=num[i]/10;
            num[i]%=10;
        }
        int index=0;
        while(index<l1+l2&&num[index]==0)index++;
        for(;index<l1+l2;index++)
            res+=num[index]+'0';
        return res;
    }