Returns all string permutations using the Heap's algorithm (linear).

GetPermutationsHeapAlg.cs

GetPermutations(['a', 'b', 'c', 'd']).Dump();

static IEnumerable<string> GetPermutations(char[] chars)
{
    yield return new string(chars);

    if (chars.Length < 2)
        yield break;

    var counter = new int[chars.Length];

    var i = 0;
    while (i < chars.Length)
    {
        if (counter[i] < i)
        {
            if (i % 2 == 0)
                (chars[0], chars[i]) = (chars[i], chars[0]);
            else
                (chars[counter[i]], chars[i]) = (chars[i], chars[counter[i]]);

            yield return new string(chars);

            counter[i]++;
            i = 1;
        }
        else
        {
            counter[i] = 0;
            i++;
        }
    }
}