using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

​

public class Program {

  public static void Main() {

    string val = "foobarfoobar";

    Console.WriteLine(string.Join(" ", FindSubstring(val, new [] {

      "foo",

      "bar"

    })));  

    //Console.WriteLine(FindSubstring(val, new [] { "foo", "bar" }));

  }

​

  static IList < int > FindSubstring(string s, string[] words) {

    var list = new List < IList < string >> ();

    DoPermutation(words, 0, words.Length - 1, list);

​

    return TrackIndexes(s, list);

  }

​

  static IList < IList < string >> DoPermutation(string[] str, int start, int end, IList < IList < string >> list) {

    if (start == end) {

      list.Add(new List < string > (str));

    } else {

      for (var i = start; i <= end; i++) {

        Swap(ref str[start], ref str[i]);

​

        DoPermutation(str, start + 1, end, list);

​

        Swap(ref str[start], ref str[i]);

      }

    }

​

    return list;

  }

​

  static void Swap(ref string a, ref string b) {

    var temp = a;

    a = b;

    b = temp;

  }

​

  static List < int > TrackIndexes(string pattern, IList < IList < string >> aLst) {

    int count = 0;

​

    List < string > strings;

    List < int > indexes = new List < int > ();

​

    foreach(var item in aLst) {

      string str = "";

​

      strings = new List < string > {

        String.Concat(item)

      };

​

      foreach(var list in item) {

        str += list;

​

        if (str.Length == strings[0].Length && pattern.Contains(str)) {

          indexes.AddRange(AllIndexesOf(pattern, str));

          count += 1;

        }

      }

    }

​

    indexes.Sort();

    return indexes.Distinct().ToList();

  }

 public static int[] AllIndexesOf(string str, string substr, bool ignoreCase = false) 

 {

  if (string.IsNullOrWhiteSpace(str) || string.IsNullOrWhiteSpace(substr)) 

  {

    throw new ArgumentException("String or substring is not specified.");

  }

​

  var indexes = new List<int>();

  int index = 0;

​

  while ((index = str.IndexOf(substr, index, ignoreCase ? StringComparison.OrdinalIgnoreCase : StringComparison.Ordinal)) != -1) 

  {

    indexes.Add(index++);

  }

​

  return indexes.ToArray();

 }

}