DSA interview question

Binary Tree Representation in Java

Easy · Data structures · Topics: tree, java

The problem

A Java service receives binary trees from a web client as level-order arrays with nulls (the format LeetCode uses, like [1, 2, 3, null, 5]) and must turn them into real linked nodes, then send results back in the same format. Write the Node class (value plus left and right references), a build method that constructs the tree from the level-order array with a queue, and a serialize method that writes a tree back out in level order, trimming trailing nulls. It's the everyday plumbing behind every tree problem.

Example: [1, 2, 3, null, 5] → a tree whose preorder is 1 2 5 3, and which serialises back to [1, 2, 3, null, 5].

Function signature

static class Node { int data; Node left, right; }
static Node build(Integer[] level);           // null marks a missing child
static List<Integer> serialize(Node root);    // level order, trailing nulls removed

Test cases

Input (level order)	Output	Explanation
`[1, 2, 3, null, 5]`	`[1, 2, 3, null, 5] preorder [1, 2, 5, 3]`	2 has only a right child
`[]`	`[] preorder []`	Empty tree
`[1, null, 2, 3]`	`[1, null, 2, 3] preorder [1, 2, 3]`
`[5]`	`[5] preorder [5]`
`[1, 2, 3, 4, 5, 6, 7]`	`[1, 2, 3, 4, 5, 6, 7] preorder [1, 2, 4, 5, 3, 6, 7]`	A perfect tree

(Confirm: children of a null are not listed in the array (LeetCode convention); serialisation removes trailing nulls so it round-trips exactly.)

How interviewers phrase it

"Show how you'd represent a binary tree in Java, and write code to build one from its level-order array (with nulls) and print it back."

Clarifying questions to ask

Which input format? (A LeetCode-style level order, where null children of null nodes are omitted.)
Should serialisation trim trailing nulls? (Yes, to round-trip exactly.)
Are values int or generic? (int here; generics are a small change.)

Follow-ups to expect

"Why a queue when building?"
"Why can't ArrayDeque hold the nulls during serialisation?"
"Array representation instead of nodes?"
"How would you serialise with preorder instead?"
"Complexity?"

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