C#

var test5 = new Dictionary<string. List<string>>>();

test5.Add("a", new List<string>{"c", "d", "b" });

test5.Add("b", new List<string> { "c","f","g", "e" });

test5.Add("e", new List<string> { "g", "h", "k" });

test5.Add("k", new List<string> { "b", "l",});

private List<T1> Result, Visited;
private T1 Start, Target;
private bool found = false;

public List<T1> FindPath(T1 start, T1 target)
{
    Result = new List<T1>();
    Visited = new List<T1>();

    Start = start;

    if (RDictT1T2[Start].Count == 0) return null;

    Target = target;

    Visited.Add(Start);
    Result.Add(Start);

    // check for direct linkage
    if (RDictT1T2.ContainsKey(Start) && RDictT1T2[Start].Contains(Target))
    {
        Result.Add(Target);
        return Result;
    }

    // recursion time
    found = false;
    return FindRecurse(Start);
}

private List<T1> FindRecurse(T1 rstart)
{
    foreach (var t1 in RDictT1T2[rstart])
    {
        if (Visited.Contains(t1)) 
        {
            continue;
        }
        else
        {
            if(
                !RDictT1T2.ContainsKey(t1)
                ||
                RDictT1T2[t1].Count == 0
              )
            {
                Visited.Add(t1);
                continue;
            }
        }

        Result.Add(t1);
        Visited.Add(t1);

        if (RDictT1T2[t1].Contains(Target))
        {
            Result.Add(Target);
            found = true;
        }
        else
        {
            FindRecurse(t1);
        }
    }

    return found ? Result : null;
}

var r1 = test5.FindPath("a", "d"); // a d

var r2 = test5.FindPath("a", "f"); // a b f

var r3 = test5.FindPath("a", "k"); // a b e k

var r4 = test5.FindPath("k", "x"); // null

var r5 = test5.FindPath("k", "g"); // k b g