17. Letter Combinations of a Phone Number

 1 static int wing=[]()
 2 {
 3     std::ios::sync_with_stdio(false);
 4     cin.tie(NULL);
 5     return 0;
 6 }();
 7 
 8 class Solution 
 9 {
10 public:
11     vector<string> letterCombinations(string digits) 
12     {
13         vector<string> res;
14         if(digits.empty())
15             return res;
16         static vector<string> v{"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
17         res.push_back("");
18         int lendigits=digits.length();
19         for(int i=0;i<lendigits;i++)
20         {
21             int num=digits[i]-‘0‘;
22             if(num<0||num>9)
23                 break; 
24             const string &candidate=v[num];
25             int lencan=candidate.length();
26             int lenres=res.size();
27             if(candidate.empty())
28                 continue;
29             vector<string> cur;
30             for(int j=0;j<lencan;j++)
31             {
32                 for(int k=0;k<lenres;k++)
33                 {
34                     string kk=res[k];
35                     kk.push_back(candidate[j]);
36                     cur.push_back(kk);
37                 }
38             }
39             res.swap(cur);
40         }
41         return res;   
42     }
43 };