Is there a memory-efficient solution to dynamic index count in a mesh?

I am implementing a ROAM terrain algorithm which, by it's nature, generates a mesh with frequently changing vertex/index count.

With vertices, I can store an array of the largest used count, and ignore any unused vertices.

With indices, however, there's no way to set the count when applying them to the mesh... you have to have an exact-count array, which requires me to instantiate one every time I apply indices to the mesh.

I know in most DirectX wrappers, you can send an array with an additional count int that tells it how many to actually use. If this were the case, I could work like vertices, where I just have an array big enough for the maximum size. Is there something like this in Unity that I'm not seeing?

For example, this is what I currently have:

 int[] Indices;
 List<int> indices = new List<int>();
 for (int i = 0; i < NextTriangleID; i++)
 {
     if (Triangles.ContainsKey(i))
     {
         Triangle t = Triangles[i];
         if (t.CT0 < 0 && t.CT1 < 0)
         {
             indices.Add(t.V0);
             indices.Add(t.V1);
             indices.Add(t.V2);
         }
     }
 }
 Indices = indices.ToArray();
 Mesh.SetIndices(Indices, MeshTopology.Triangles, 0);

And this is what I'd like to have:

 int[] Indices;
 int IndexCount = 0;
 for (int i = 0; i < NextTriangleID; i++)
 {
     if (Triangles.ContainsKey(i))
     {
         Triangle t = Triangles[i];
         if (t.CT0 < 0 && t.CT1 < 0)
         {
             Indices[IndexCount] = t.V0;
             IndexCount++;
             Indices[IndexCount] = t.V1;
             IndexCount++;
             Indices[IndexCount] = t.V2;
             IndexCount++;
         }
     }
 }
 Mesh.SetIndices(Indices, IndexCount, MeshTopology.Triangles, 0);