【leetcode】Letter Combinations of a Phone Number

Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"

Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

 

回溯法递归调用即可：

 1 class Solution {

 2 

 3 public:

 4 

 5     vector<string> letterCombinations(string digits) {

 6 

 7         vector<string> num2string(10);

 8 

 9         num2string[0]="";

10 

11         num2string[1] = "";  

12 

13         num2string[2] = "abc";  

14 

15         num2string[3] = "def";  

16 

17         num2string[4] = "ghi";  

18 

19         num2string[5] = "jkl";  

20 

21         num2string[6] = "mno";  

22 

23         num2string[7] = "pqrs";  

24 

25         num2string[8] = "tuv";  

26 

27         num2string[9] = "wxyz";

28 

29        

30 

31         vector<string> result;

32 

33         string tmp="";

34 

35         combination(digits,0,tmp,num2string,result);

36 

37         return result;

38 

39     }

40 

41    

42 

43    

44 

45     void combination(string &digits,int index,string tmp,vector<string> &num2string,vector<string> &result)

46 

47     {

48 

49         if(index==digits.length())

50 

51         {

52 

53             result.push_back(tmp);

54 

55             return;

56 

57         }

58 

59        

60 

61         string mapString=num2string[digits[index]-'0'];

62 

63         for(int i=0;i<mapString.length();i++)

64 

65         {

66 

67             tmp.push_back(mapString[i]);

68 

69             combination(digits,index+1,tmp,num2string,result);

70 

71             tmp.pop_back();

72 

73         }

74 

75     }

76 

77 };