String Permutations

public static IEnumerable<IEnumerable<T>> GetPermutations<T>(IReadOnlyCollection<T> list, int length)
        {
            if (length == 1) return list.Select(t => new[] { t });

            return GetPermutations(list, length - 1)
                .SelectMany(t => list.Where(e => !t.Contains(e)),
                    (t1, t2) => t1.Concat(new[] { t2 }));
        }