# GATE | GATE-CS-2004 | Question 35

Consider the label sequences obtained by the following pairs of traversals on a labeled binary tree. Which of these pairs identify a tree uniquely ?

```(i)     preorder and postorder
(ii)    inorder and postorder
(iii)   preorder and inorder
(iv)   level order and postorder```

(A) (i) only
(B) (ii), (iii)
(C) (iii) only
(D) (iv) only

Explanation: Here, we consider each and every option to check whether it is true or false.

1) Preorder and postorder

For the above trees,

Preorder is AB

Postorder is BA

It shows that preorder and postorder can’t identify a tree uniquely.

2) Inorder and postorder define a tree uniquely

3) Preorder and Inorder also define a tree uniquely

4) Levelorder and postorder can’t define a tree uniquely. For the above example,

Level order is AB

Postorder is BA

This solution is contributed by Anil Saikrishna Devarasetty

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