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;
