模板：大整数减法

string sub(string s1,string s2)
{
    int flag = 0;
    if(s1.length()<s2.length() || (s1.length()==s2.length() && s1 < s2))
    {
        flag = 1;
        string temp=s1;
        s1=s2;
        s2=temp;
    }


    int i,j;
    for(i=s1.length()-1,j=s2.length()-1;i>=0;i--,j--)
    {
        s1[i]=char(s1[i]-(j>=0?s2[j]-'0':0));   //注意细节
        if(s1[i]-'0'<0)
        {
            s1[i]=char(s1[i] + 10);
            s1[i-1]--;
        }
    }

    for(i = 0; i < s1.length(); i++)
    {
        if(s1[i] != '0') break;
    }
    if(i == s1.length()) i = s1.length()-1;
    s1 = s1.substr(i);

    if(flag) s1 = "-" + s1;

    return s1;
}