LeetCode Online Judge 题目C# 练习 - 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.

 

 1         public static List<string> LetterCombinationsofaPhoneNumber(string digits)

 2         {

 3             List<string>[] ret = new List<string>[]

 4                 {

 5                     new List<string>(),

 6                     new List<string>()

 7                 };

 8             if (digits.Length == 0)

 9                 return ret[0];

10 

11             //mapping of digit to letters

12             Dictionary<char, string[]> map = new Dictionary<char, string[]>();

13             map.Add('2', new string[]{"a", "b", "c"});

14             map.Add('3', new string[]{"d", "e", "f"});

15             map.Add('4', new string[]{"g", "h", "i"});

16             map.Add('5', new string[]{"j", "k", "l"});

17             map.Add('6', new string[]{"m", "n", "o"});

18             map.Add('7', new string[]{"p", "q", "r", "s"});

19             map.Add('8', new string[]{"t", "u", "v"});

20             map.Add('9', new string[]{"w", "x", "y", "z"});

21 

22             int curr_index = 0;

23             int new_index = 1;

24             //set up the first List<string>

25             foreach (var newchar in map[digits[0]])

26             {

27                 ret[curr_index].Add(newchar);

28             }

29             

30             //loop the rest of digits

31             for (int i = 1; i < digits.Length; i++)

32             {

33                 //construct next List<string> with current List<string> and current digit

34                 foreach (var pre in ret[curr_index])

35                 {

36                     foreach (var newchar in map[digits[i]])

37                     {

38                         ret[new_index].Add(pre + newchar);

39                     }

40                 }

41 

42                 ret[curr_index].Clear();

43                 curr_index = new_index;

44                 new_index = (new_index + 1) % 2;

45             }

46 

47             return ret[curr_index];

48         }

代码分析：

　　建立两个List<string>，替换使用，通过前一个List<string>的每一个元素后面 + map[digit] 的每一个字母，产生后一个List<string>。

　　例如： “49”

　　List<string> 1 : g, h, i;

　　List<string> 2 : gw, gx, gy, gz, hw, hx, hy, hz, iw, ix, iy, iz;