C＃遞歸循環

Question

我有這些數據

string [] letters = {"a","b","c"}; 

我想要的功能等，generateString（1） 如果值爲1，則輸出爲：

a 
b 
c 

如果generateString（3），輸出

a 
b 
c 
aa 
ab 
ac 
ba 
bb 
bc 
ca 
cb 
cc 
aaa 
aab 
aac 
aba 
abb 
abc 
aca and so on... 

我可以像這樣做，

foreach(var a in data){ 
foreach(var b in data){ 
    foreach(var c in data){ 
    Console.WriteLine(a + b + c); 
    } 
} 
} 

它只能生成高達3，如果我想使它4，然後我添加另一個foreach，這是我認爲不是一個更好的主意。

有什麼建議嗎？

Answer 1

static string[] generateString(string[] letters, int len) 
{ 
    if (len < 0) throw new ArgumentOutOfRangeException("length can't be less than zero."); 
    switch (len) 
    { 
     case 0: return new string[0]; 
     case 1: return letters; 
     default: 
      // all possible combinations which are shorter than required 
      // recursion is used here 
      var shorter_x = generateString(letters, len - 1).ToArray(); 

      // all combinations which have length = len - 1 
      var shorter_1 = shorter_x.Where(line => line.Length == len - 1).ToArray(); 

      // resulting array 
      return shorter_x.Union(letters.SelectMany(letter => shorter_1.Select(shorter => letter + shorter))).ToArray(); 
    } 
} 

UPD：代表數據

char[] letters = { 'a', 'b', 'c' } 

會更一致。在這種情況下，函數看起來像

static string[] generateString(char[] letters, int len) 
{ 
    if (len < 0) throw new ArgumentOutOfRangeException("length can't be less than zero."); 
    switch (len) 
    { 
     case 0: return new string[0]; 
     case 1: return letters.Select(char.ToString).ToArray(); 
     default: 
      ...... 

Answer 2

它看起來像成才這樣

List<List<String>> foo(int i){ 
    if(i==1){ 
     return new List<List<String>>(new List<String>("a", "b", "c")); 
    } else{ 
     var result = new List<List<String>>(); 
     foreach(List<String> list in foo(i-1)){ 
      foreach(String elem in new String[]{"a", "b", "c"}){ 
       var tmp = new List<String>(list); 
       tmp.Add(elem); 
       result.Add(tmp); 
      } 
     } 
     return result; 
    } 
} 

我不知道sintax和列表構造函數，但在總體思路所示