Given an array of letters, find all combinations that contain all the letters in the array

GetCombinations

        public static List<string> GetSubCombinations(List<string> letters)
        {
            if(letters.Count() == 1) return letters;

            var combinations = new List<string>();
            for(int i=0; i< letters.Count; i++)
            {
                var letterToInterleave = letters[i]; 

                var remainder = letters.Where(x => x != letterToInterleave).ToList();
            
                if (remainder.Count() == 1)
                {
                    combinations.Add(letterToInterleave + remainder[0]);
                }
                else
                {
                    var subPerms = GetSubCombinations(remainder);
                    foreach (var s in subPerms)
                    {
                        combinations.Add(letterToInterleave + s);
                    }
                }


            }
        

  	    return combinations;
        }