大数相加模板

string sum(string s1,string s2)

{

    if(s1.length()<s2.length())

    {

        string temp=s1;

        s1=s2;

        s2=temp;

    }

    int i=s1.length()-1,j=s2.length()-1;

    for(;i>=0;i--,j--)

    {

        s1[i]=char(s1[i]+(j>=0?s2[j]-'0':0));

        if(s1[i]-'0'>=10)

        {

            s1[i]=char((s1[i]-'0')%10+'0');

            if(i)

                s1[i-1]++;

            else

                s1='1'+s1;

        }

    }

    return s1;

}