模板：大整数乘法

 1 string mul(string str1,string str2)
 2 {
 3     vector<int> v_res(str1.size()+str2.size(),0);
 4     string::size_type i,j;
 5     vector<int>::size_type k,p;
 6 
 7     reverse(str1.begin(),str1.end());
 8     reverse(str2.begin(),str2.end());
 9     for(i = 0; i != str1.size(); ++i)
10     {
11         for(j = 0; j != str2.size(); ++j)
12         {
13             v_res[i+j] += (str1[i]-‘0‘) * (str2[j] - ‘0‘);
14         }
15     }
16     for(k = 0; k != v_res.size() - 1; ++k)
17     {
18         v_res[k+1] += v_res[k] / 10;
19         v_res[k] = v_res[k] % 10;
20     }
21 
22     for(p = v_res.size() - 1; p != -1; --p)
23     {
24         if(v_res[p] != 0) break;
25     }
26     if(p == -1) p = 0;
27 
28     string s_res(p+1,‘0‘);
29     for(k = p; k != -1; --k) s_res[p-k] = char(v_res[k] + ‘0‘);
30     
31 
32     return s_res;
33 
34 }