Which Tree traversal String is unique?
Preorder traversal:

This traversals always yield unique binary trees. (proof for this remains. To me it seems that it is correct due to procedure to construct tree that follows. However it might not be the case.)

For instance consider traversal sequence $6,5,N,N,5,3,N,N,2,N,N$.

We can proceed as follow to construct binary tree.


  
  First symbol is always root of tree.
  


Root will always be identified uniquely.


  
  Try to parse remaining part of input recursively(This will be our left sub-tree). When one sub-tree is parsed  return.
  


Here this left sub-tree parsing will start with input $5,N,N,5,3,N,N,2,N,N$ and will return after input $5,3,N,N,2,N,N$ left to be processed.


  
  Try to parse yet remaining part of input recursively(This will be our right sub-tree).
  




Post-order traversal: This is similar to pre-order.



inorder traversal: This does not always yield unique binary tree. Here is counterexample.

Consider preorder traversal $N,5,N,6,N,3,N,5,N,2,N$. And following two binary trees.

        6                    5
       /  \                /  \
      5    5              6    2
     / \  /  \           / \  / \
    N  N 3    2         5   3 N  N
        / \  / \       / \ / \
       N   N N  N     N  N N  N