# The number of Binary Search Tree that exist with same Postorder and Inorder

how many BST exist with same postorder and inorder traversal?

I know that in binary tree (Not BST), it is one.

but i have a book from that said for BST it is CATALAN number. i become confused.

Your confusion might stem from this: The number of binary trees on $n$ vertices is $C_n$, the $n$-th Catalan number. Since in a BST, the left-right order of vertices is fixed, the number of BSTs with a given inorder traversal is also $C_n$. However, with the added restriction that the postorder traversal must be the same as the inorder traversal, the BST can only have left branches.
In more detail, recall that inorder = left-visit-right and postorder = left-right-visit.