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;
}