core-pattern

Tree Depth-First Search

Define what each subtree returns upward or carries downward to solve traversal, validation, aggregation, and structural tree problems.

Problems

Problems in Tree Depth-First Search

Practice problems that share this primary solution pattern and compare the clues that reveal it.

Smartphone displaying AI app with book on AI technology in background.

beginner

10 min read

Binary Tree Inorder Traversal

The difficult part of inorder traversal is not remembering “left, node, right.” It is preserving the parent node while the left subtree is still…

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Detailed view of a sewing machine needle stitching fabric, highlighting sewing process and textile technology.

advanced

12 min read

Recover Binary Search Tree

A binary search tree has a structural invariant that is more useful than its parent-child relationships: inorder traversal visits values in sorted order.…

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Bright geometric pattern on a fence with green and white circles against a blue sky.

beginner

10 min read

Same Tree

A tree comparison fails the moment you forget that a missing child is also part of the structure.

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Close-up of ornate Corinthian columns on ancient Roman architecture, showcasing intricate carvings and historical craftsmanship.

intermediate

11 min read

Validate Binary Search Tree

A BST validator must remember the ancestors that still constrain the current node.

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Dynamic abstract image of vibrant green light trails in motion, creating a futuristic feel.

beginner

10 min read

Symmetric Tree

The key to the Symmetric Tree solution is to compare two nodes as a mirror pair, not to traverse the left and right subtrees independently.

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Technician working on humanoid robot at a tech exhibition in Guimaraes, Portugal.

intermediate

10 min read

Binary Tree Level Order Traversal

If you visit every node but mix adjacent depths, the traversal is still wrong. The key is to make the queue represent one level at a time.

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Lush green tropical leaves with dewy surface, perfect for nature and botanical themes.

intermediate

9 min read

Binary Tree Zigzag Level Order Traversal

The queue does not need to zigzag. It only needs to preserve a predictable order while we place each value into its correct output position.

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Dynamic abstract depiction of digital circuits with vivid lights and glowing lines.

beginner

9 min read

Maximum Depth of Binary Tree

The cleanest way to solve this problem is to stop counting depth globally. Ask each subtree for its answer, then let the parent combine those answers.

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Close-up view of the historic Colosseum in Rome, Italy at sunset, showcasing ancient architecture.

intermediate

11 min read

Binary Tree Level Order Traversal II

The output is bottom-up, but the traversal should still be top-down: collect ordinary BFS levels, then reverse the completed groups.

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Telecommunication antennas on a rooftop in Istanbul, showcasing modern urban infrastructure.

intermediate

12 min read

Convert Sorted List to Binary Search Tree

A sorted array gives you the middle value immediately. A singly linked list makes you walk for it. The better solution avoids the walk entirely: let…

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Close-up of a craftsman using an angle grinder, emitting sparks in a metal workshop.

beginner

9 min read

Balanced Binary Tree

A reliable Balanced Binary Tree solution carries two facts upward from every subtree: its height and whether it is balanced.

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Students engaged in study at a computer lab, focused on technology and teamwork.

beginner

10 min read

Minimum Depth of Binary Tree

The shortest path counts only when it reaches a leaf. That one condition is what breaks the tempting min(left, right) solution.

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A top view on charts and smartphone in an office, showcasing data analytics.

beginner

10 min read

Path Sum

A matching sum at an internal node is not enough. The path must start at the root, end at a leaf, and satisfy the target at that boundary.

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Wide-angle view of a towering sand dune against a vibrant blue sky, showcasing the beauty of desert landscapes.

intermediate

10 min read

Path Sum II

That is the central trap in Path Sum II. An internal node may bring the running sum to targetSum, but its path is still incomplete if the node has a child.…

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Beautiful handmade terracotta pots displayed on shelves in an outdoor market, showcasing artisanal craftsmanship.

intermediate

10 min read

Flatten Binary Tree to Linked List

It places the left subtree in the correct position, but it can disconnect the original right subtree. The real task is to move the left subtree in front of…

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View of the ancient Odeon of Herodes Atticus in Athens with a cityscape backdrop.

intermediate

12 min read

Populating Next Right Pointers in Each Node

The queue-based solution is easy to see. The constant-space solution is easier to miss: once one level is connected, its next pointers become the queue for…

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A peaceful upward view of green tree branches extending towards a vibrant blue sky in spring.

intermediate

10 min read

Populating Next Right Pointers in Each Node II

The queue solution is easy to derive. The constant-space solution comes from reusing the next pointers you are building as the queue for the next level.

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