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;
}