Adding and multiplying large numbers quickly

Tarsus adding:

 1 string sum(string s1,string s2) { 
 2     if(s1.length() < s2.length()) {string temp = s1;s1 = s2; s2 = temp;}
 3     int i,j;
 4     for(i = s1.length() - 1,j = s2.length() - 1;i >= 0;i--,j--){
 5         s1[i] = char(s1[i] + (j >= 0?s2[j] - '0':0));
 6         if(s1[i] - '0' >= 10){
 7             s1[i] = char((s1[i] - '0') % 10 + '0');
 8             if(i) s1[i-1] ++;
 9             else s1 = '1' + s1;
10         }
11     }
12     return s1;
13 }

 

Quick multiplied

1 ll ksc(l a, ll b, ll mod)    //快速乘法
2 {
3     return (a*b-(ll)((long double)a*b/mod)*mod+mod)%mod;
4 }