ExPermutations2.cs

ExPermutations2.cs

// 辞書順で順列列挙

using System;
using System.Linq;
using System.Collections.Generic;

static class Ex {
  
  private static IEnumerable<IEnumerable<T>[]> __MoveRtoL<T>(IEnumerable<T> e1, IEnumerable<T> e2) {
    while(e2.Count() > 0) {
      yield return new IEnumerable<T>[]{e1, e2};
      e1 = e1.Concat(e2.Take(1));
      e2 = e2.Skip(1);
    }
  }

  public static IEnumerable<IEnumerable<T>> Permutations<T>(this IEnumerable<T> xs) {
    if(xs.Count() == 0) yield return Enumerable.Empty<T>();
    foreach(var ea in __MoveRtoL(Enumerable.Empty<T>(), xs)) {
      var ys = ea[1].Take(1);
      foreach(var zs in ea[0].Concat(ea[1].Skip(1)).Permutations()) {
        yield return ys.Concat(zs);
      }
    }
  }
}

// Examples

class Program {
  
  public static void Main(string[] args) {

    foreach(var xs in Enumerable.Range(1,3).Permutations()) {
      Console.WriteLine(string.Join("-", xs.Select(x=>x.ToString()).ToArray()));
    }

    foreach(var xs in "ABCD".Permutations()) {
      Console.WriteLine(string.Join("-", xs.Select(x=>x.ToString()).ToArray()));
    }
  }
}