Question

How to find preorder predecessor,preorder successor, post-order predecessor and post-order successor

Answer 1

If you know what inorder is then predecessor / successor is too easy 
e.g  inorder of a BST  is increasing sorted order 
3   5   8   12   15   21   25   27   31   35
if u want to find inorder successor of 15  
u can see it is 21  
Now how to find it in BST   inorder successor of 15
pick up right subtree of 15 and find minimum element there which is 21 
And to minimum element in BST you have to travel to leftmost branch upto leave i.e go left left left upto leave
Overall Conclusion :  1. Pick right subtree
                                2. then go left left left ..... find minimum element in that subtree which is leftmost element

If you understand this predecessor is just reverse of it 
lemme know if you still have problem

Comments

But the question which I asked is pre-order and post-order's predecessor and successor