DSA topic · Stage 3

Trees interview preparation

Reason recursively about hierarchical structure and subtree information.

DSA prep Practice 7 questions

Concept

Tree interviews combine traversal choice with a precise invariant returned from each subtree.

Complexity lens

Count each visited node and include recursion or explicit-stack height in space analysis.

Common mistakes

Confusing node count with tree height
Missing null roots
Using global state unnecessarily

Recognition

Decide from constraints, not labels.

These clues narrow the model. They do not replace an invariant or complexity check.

Recognition clues

The answer composes from subtrees
Level, depth, ancestor, or ordered-tree constraints matter
There is one unique path from root to a node

Implementation options

Recursive DFS with an explicit return meaning
Iterative DFS with a stack
BFS for levels or nearest depth

Interview behavior

Define the base case and subtree contract first
Distinguish node count, height, and width in complexity
Avoid hidden global state when a return value is sufficient

Patterns that operate on this topic

DFSEach active frame owns one node and the unfinished neighbors or child results needed before that frame can resolve.BFSWhen a node is first dequeued, its recorded layer is the minimum number of unweighted edges from the source set.TrieA path from root spells a prefix present in inserted keys; a terminal marker distinguishes a complete key from a prefix only.

Representative problems

7 current Trees questions.

Exact active distribution: 4 easy, 3 medium, 0 hard. External records contain metadata and original guidance only.

External catalogUnverified
Maximum Depth of Binary Tree
Compare recursive subtree aggregation with level-order traversal.
Trees DFS BFS
EasyFree / publicSolve problem
External catalogUnverified
Binary Tree Level Order Traversal
Capture each queue level before adding the next one.
Trees Stacks & Queues BFS
MediumFree / publicSolve problem
External catalogUnverified
Validate Binary Search Tree
Carry valid ranges rather than comparing only parent and child.
Trees Binary Search DFS
MediumFree / publicSolve problem
External catalogUnverified
Implement Trie (Prefix Tree)
Distinguish a complete word from a reachable prefix.
Tries Strings Trees Trie
MediumFree / publicSolve problem
External catalogUnverified
Invert Binary Tree
Practices a simple mutation at every node with clear recursion boundaries.
Trees DFS
EasyFree / publicSolve problem
External catalogUnverified
Path Sum
Distinguishes carrying path state into a subtree from returning aggregate state.
Trees DFS
EasyFree / publicSolve problem
External catalogUnverified
Diameter of Binary Tree
Separates the value returned upward from the global answer updated at a node.
Trees DFS
EasyFree / publicSolve problem

Review state

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Review prompts

What did each recursive call return?
Could depth exceed safe recursion?
Did I handle the null root?

Prerequisites and next step

Graph Thinking

Model relationships explicitly and reason about reachability, order, components, grids, and weighted paths.

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