大数计算(加减乘除)

 1 string multiply(string num1, string num2) {
 2     int len1 = num1.size();
 3     int len2 = num2.size();
 4     int c = 0;
 5     string tmp(len1+len2,‘0‘);
 6     for(int i=len2-1;i>=0;i--)
 7     {
 8         int b = num2[i]-‘0‘;
 9         for(int j=len1-1;j>=0;j--)
10         {
11             int a = num1[j]-‘0‘;
12             int d = tmp[i+j+1]-‘0‘;
13             int x = a*b+d+c;
14             tmp[i+j+1] = x%10 +‘0‘;
15             c = x/10;
16         }
17         if(c)
18         {
19             tmp[i] = c+‘0‘;
20             c=0;
21         }
22     }
23     int k=0;
24     for(;tmp[k]==‘0‘&&k<tmp.size();k++);
25     if(k==tmp.size())return "0";
26     else return tmp.substr(k);
27   }