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Find Duplicate Subtrees — Serialize + HashMap
Find all duplicate subtrees by serializing each subtree and tracking serialization counts in a hash map.
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Tree
26 articles
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Flatten Binary Tree to Linked List — Morris Traversal
Flatten a binary tree in-place into a linked list following pre-order using the Morris traversal approach.
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Serialize and Deserialize Binary Tree — BFS Level Order
Serialize a binary tree to a string using BFS level-order and deserialize back using a queue for reconstruction.
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Trees — Complete Guide: All Patterns & Problem Index
Master every tree pattern: DFS traversals, BFS level-order, BST operations, LCA, path sum, and serialization — with templates and full index of 75 problems.
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Maximum Depth of Binary Tree — DFS and BFS Approaches
Find the maximum depth of a binary tree using recursive DFS (one line) or iterative BFS level counting.
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Invert Binary Tree — Recursive Swap
Invert a binary tree by recursively swapping left and right children at every node.
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Symmetric Tree — Mirror Recursion
Check if a binary tree is symmetric by comparing mirror subtrees with a two-pointer recursive approach.
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Path Sum — Root-to-Leaf DFS with Running Remainder
Determine if a root-to-leaf path exists with a given sum using DFS that subtracts each node value from the target.
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Same Tree — Structural and Value Comparison
Check if two binary trees are identical by recursively comparing structure and node values.
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Balanced Binary Tree — Height DFS with Early Exit
Check if a binary tree is height-balanced by computing subtree heights and returning -1 early on imbalance.
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Merge Two Binary Trees — Recursive Overlay
Merge two binary trees by adding overlapping node values recursively, using existing nodes where possible.
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Range Sum of BST — BST Property Pruning DFS
Sum all BST values in [low, high] using BST property to prune entire subtrees outside the range.
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Search in a Binary Search Tree — BST Property Navigation
Find a node with given value in a BST by navigating left or right based on the BST property.
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Binary Tree Level Order Traversal — BFS Queue
Return values grouped by level using BFS with a queue, processing each level in a batch loop.
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Binary Tree Zigzag Level Order — Alternating Direction BFS
Traverse a binary tree in zigzag level order by toggling insertion direction at each level.
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Binary Tree Right Side View — BFS Last Element Per Level
Find the rightmost node at each level using BFS and taking the last element seen at each depth.
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Path Sum II — DFS Backtracking All Paths
Collect all root-to-leaf paths with a given sum using DFS backtracking, appending and removing the current node.
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Diameter of Binary Tree — Tree DP Height Trick
Find the diameter (longest path between any two nodes) using a depth function that tracks the maximum path through each node.
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Validate Binary Search Tree — Min/Max Bounds DFS
Validate a BST by passing down min and max bounds through DFS, ensuring every node falls strictly within its valid range.
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Kth Smallest Element in a BST — Inorder Traversal
Find the kth smallest element in a BST using inorder traversal (left-root-right) which visits nodes in sorted order.
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Lowest Common Ancestor of Binary Tree — Post-Order DFS
Find the LCA of two nodes using post-order DFS: return root when found, propagate upward and split at the ancestor.
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Lowest Common Ancestor of BST — BST Property Navigation
Find the LCA in a BST in O(h) by navigating left or right based on whether both nodes are in the same subtree.
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Construct Binary Tree from Preorder and Inorder — HashMap Index
Build a binary tree from preorder and inorder traversals by using the preorder root and inorder index to split left/right subtrees.
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Path Sum III — Prefix Sum HashMap on Tree
Count paths in a binary tree summing to target (not necessarily root-to-leaf) using a prefix sum frequency map.
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Meta — Serialize and Deserialize Binary Tree (BFS/Preorder)
Serialize a binary tree to a string and deserialize it back. Meta loves this problem for testing tree traversal and string parsing. Both BFS and preorder DFS approaches shown.
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Meta — Binary Tree Right Side View (BFS Level Order)
Return the values visible from the right side of a binary tree — the last node of each BFS level. O(n) BFS with level tracking.
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