Merge Two Binary Tree- LeetCode
This is a very interesting problem! 

Firstly, you have correctly identified the 3 possible states:


both tree1 and tree2 are null
tree1 or tree2 is null
both tree1 and tree2 are not null


That being said, I would recommend a couple of things:


Break up the problem into 4 possible states (i.e., don't handle the case when tree1 or tree2 is null in one conditional statement.
if passing in the merged tree tree3 to doMerge in order to modify it in the method, you should utilize C#'s out parameter.
Why do you need the Direction enum? It seems to make the code harder to understand.


Here is my implementation of doMerge:

public TreeNode MergeTrees(TreeNode t1, TreeNode t2) 
{
    if (t1 == null &amp;&amp; t2 == null) return null;
    if (t1 != null &amp;&amp; t2 == null) return t1;
    if (t1 == null &amp;&amp; t2 != null) return t2;
    TreeNode merged = new TreeNode(t1.val + t2.val);
    merged.left = MergeTrees(t1.left, t2.left);
    merged.right = MergeTrees(t1.right, t2.right);
    return merged;
}