模板：大整数减法

 1 string sub(string s1,string s2)
 2 {
 3     int flag = 0;
 4     if(s1.length()<s2.length() || (s1.length()==s2.length() && s1 < s2))
 5     {
 6         flag = 1;
 7         string temp=s1;
 8         s1=s2;
 9         s2=temp;
10     }
11 
12 
13     int i,j;
14     for(i=s1.length()-1,j=s2.length()-1;i>=0;i--,j--)
15     {
16         s1[i]=char(s1[i]-(j>=0?s2[j]-'0':0));   //注意细节
17         if(s1[i]-'0'<0)
18         {
19             s1[i]=char(s1[i] + 10);
20             s1[i-1]--;
21         }
22     }
23 
24     for(i = 0; i < s1.length(); i++)
25     {
26         if(s1[i] != '0') break;
27     }
28     if(i == s1.length()) i = s1.length()-1;
29     s1 = s1.substr(i);
30 
31     if(flag) s1 = "-" + s1;
32 
33     return s1;
34 }