GATE CSE 2026 Session 1 Q62: Postorder position of 60 in a BST given by its preorder

The following sequence corresponds to the preorder traversal of a binary search tree $T$ :

50, 25, 13, 40, 30, 47, 75, 60, 70, 80, 77

The position of the element 60 in the postorder traversal of $T$ is ______. (answer in integer)

Note: The position begins with 1.

Try it: numerical answer: type the value

Your answer

Numerical answer. No negative marking.

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Answer: 7 (official GATE 2026 final answer key)

Concept used: A BST is determined by its preorder: the root comes first, then smaller keys, then larger keys.

First step: Rebuild the tree: 50 has left subtree {25, 13, 40, 30, 47} and right subtree {75, 60, 70, 80, 77}; then list the postorder.

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Source: official GATE 2026 Session 1 CS question paper and final answer key, organised by IIT Guwahati. Question paper · Answer key