Maximum number of distinct binary tree possible with 4 nodes

what is the maximum number of distinct binary tree is possible with 4 nodes? ans is 6 but how? acc to me it should be 14

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What is your definition of distinct? –  Aryabhata Feb 2 '12 at 19:22
means different binary tree can possible –  Ashish Feb 2 '12 at 19:23

I can think of no definitions of binary tree and distinct that would produce six distinct binary trees.

There are $14$ plane binary trees, i.e., rooted binary trees in which left and right offspring are distinguished. If left and right offspring are not distinguished, there are only three rooted binary trees:

   *                      *                      *
|                      |                     / \
*                      *                    *   *
/ \                     |                    |
*   *                    *                    *
|
*


If you consider unrooted trees, the last two of these are the same: there are only two unrooted binary trees.

If you consider labelled trees, the numbers go up considerably: unrooted the chain graph with four vertices already can be labelled in $12$ ways, so there’s no way to get just six.

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