Multiply Strings

 1 class Solution {
 2 public:
 3     string multiply(string num1, string num2) {
 4         if(num1.size()==0||num2.size()==0) return "0";
 5         int len1=num1.size();
 6         int len2=num2.size();
 7         int carry=0;
 8         string pre;
 9         for(int i=len1-1;i>=0;i--)
10         {
11             int a=num1[i]-‘0‘;
12             string cur;
13             int weishu=len1-i;
14             while(weishu>1)
15             {
16             cur.append(1,‘0‘);
17             weishu--;
18             }
19             for(int j=len2-1;j>=0;j--)
20             {
21                 int b=num2[j]-‘0‘;
22                 int t=(a*b+carry)%10;
23                 carry=(a*b+carry)/10;
24                 cur.append(1,(t+‘0‘));
25             }
26             if(carry!=0)
27             {
28                 cur.append(1,(carry+‘0‘));
29                 carry=0;
30             }
31             sumstr(pre,cur);
32             pre=cur;
33             
34         }
35         std::reverse(pre.begin(),pre.end());
36         int start=0;
37         while(pre[start]==‘0‘&&start<pre.size())
38         {
39             start++;
40         }
41         if(start==pre.size())return "0";
42         return pre.substr(start,pre.size()-start);
43     }
44     
45     void sumstr(string& pre, string& cur)
46     {
47         int len1=pre.size();
48         int len2=cur.size();//should be equal or longer than len1
49         int carry=0;
50         int i=0;
51         while(i<len1)
52         {
53             int t=(pre[i]-‘0‘+cur[i]-‘0‘+carry)%10;
54             carry=(pre[i]-‘0‘+cur[i]-‘0‘+carry)/10;
55             cur[i]=t+‘0‘;
56             i++;
57             
58         }
59         while(carry!=0&&i<len2)
60         {
61             int t=(cur[i]-‘0‘+carry)%10;
62             carry=(cur[i]-‘0‘+carry)/10;
63             cur[i]=t+‘0‘;
64             i++;
65         }
66         if(carry!=0)cur.append(1,(carry+‘0‘));
67         
68     }
69 };

1:    string multiply(string num1, string num2) {  
2:      // Start typing your C/C++ solution below  
3:      // DO NOT write int main() function  
4:      if(num1.size() ==0 || num2.size() ==0) return 0;  
5:      string res(num1.size()+num2.size()+1, ‘0‘);  
6:      std::reverse(num1.begin(), num1.end());  
7:      std::reverse(num2.begin(), num2.end());  
8:      for(int i =0; i < num1.size(); i++)  
9:      {  
10:        int dig1 = num1[i] -‘0‘;  
11:        int carry = 0;  
12:        for(int j = 0; j< num2.size(); j++)  
13:        {  
14:          int dig2 = num2[j] - ‘0‘;  
15:          int exist = res[i+j] -‘0‘;          
16:          res[i+j] = (dig1*dig2+carry+ exist) % 10 +‘0‘;    
17:          carry = (dig1*dig2+carry+exist)/10;  
18:        }  
19:        if(carry >0)  
20:        {  
21:          res[i+num2.size()] = carry + ‘0‘;  
22:        }  
23:      }  
24:      std::reverse(res.begin(), res.end());  
25:      int start =0;  
26:      while(res[start] ==‘0‘ && start < res.size())  
27:      {  
28:        start++;  
29:      }  
30:      if(start == res.size()) return "0";  
31:      return res.substr(start, res.size()-start);  
32:    }