GATE CSE 2020 Q15: Postorder of a BST from its preorder 15, 10, 12, 11, 20, 18, 16, 19

The preorder traversal of a binary search tree is 15, 10, 12, 11, 20, 18, 16, 19. Which one of the following is the postorder traversal of the tree?

Try it: multiple choice: exactly one correct option

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Answer: B (official GATE 2020 final answer key)

Concept used: In a BST the preorder fixes the tree: the first key is the root, smaller keys form the left subtree.

First step: Root 15; left subtree from 10, 12, 11 and right subtree from 20, 18, 16, 19. Rebuild each recursively.

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Source: official GATE 2020 CS question paper and final answer key, organised by IIT Delhi. Question paper · Answer key