Binary Tree

Subtitle / Summary The easiest way to get lost in LeetCode 124 is to make one recursive return value carry too much meaning. The stable design is to separate two roles: the recursion returns only the best single-side gain to the parent, while the full path through the current node is used to update the global maximum. Reading time: 12-15 min Tags: Hot100, binary tree, tree DP, DFS, postorder SEO keywords: Binary Tree Maximum Path Sum, tree DP, single-side gain, postorder, DFS, LeetCode 124 Meta description: Learn LeetCode 124 from the single-side gain invariant, global path update, and negative-branch pruning, with runnable multi-language implementations. A — Algorithm Problem Restatement A path in a binary tree is a sequence of nodes such that: ...

Subtitle / Summary LeetCode 199 is not really about visual imagination. It is about translating a viewpoint problem into a level problem. Once you realize that the right side view is simply the last node of each level, the problem becomes a standard breadth-first traversal. Reading time: 10-13 min Tags: Hot100, binary tree, BFS, level order, queue SEO keywords: Binary Tree Right Side View, BFS, level order, queue, right-first DFS, LeetCode 199 Meta description: Learn LeetCode 199 from the core equivalence “right side view = last node of each level”, with step-by-step derivation, engineering mappings, and runnable multi-language implementations. A — Algorithm Problem Restatement Given the root root of a binary tree, imagine standing on its right side and return the values of the nodes you can see from top to bottom. ...

Subtitle / Summary The key to LeetCode 105 is not memorizing that preorder and inorder can reconstruct a tree. It is understanding what each traversal contributes: preorder tells you the root, inorder tells you the left/right boundary. Once those roles are clear, the whole problem becomes a clean indexed divide-and-conquer. Reading time: 12-15 min Tags: Hot100, binary tree, divide and conquer, hash map, preorder SEO keywords: Construct Binary Tree from Preorder and Inorder Traversal, preorder, inorder, divide and conquer, hash map, LeetCode 105 Meta description: Learn LeetCode 105 from traversal roles, indexed recursion, and hash-map root lookup, with runnable multi-language implementations. A — Algorithm Problem Restatement Given the preorder traversal preorder and inorder traversal inorder of a binary tree, reconstruct the tree and return its root. ...

Subtitle / Summary The key to LeetCode 108 is not recursion by itself. It is noticing that the problem wants two things at the same time: BST ordering and height balance. Once you read those two constraints together, “pick the middle element as the root” stops being a trick and becomes the natural construction rule. Reading time: 11-14 min Tags: Hot100, binary tree, BST, divide and conquer, recursion SEO keywords: Convert Sorted Array to Binary Search Tree, BST, balanced BST, divide and conquer, recursion, LeetCode 108 Meta description: Learn LeetCode 108 from the midpoint-construction idea, with step-by-step derivation, engineering mappings, and runnable multi-language implementations. A — Algorithm Problem Restatement Given an integer array nums sorted in strictly increasing order, convert it into a height-balanced binary search tree. ...

Subtitle / Summary The real difficulty of LeetCode 114 is not “flattening” a tree. It is rewiring pointers without destroying the structure you still need. Once you notice that the final linked list must follow preorder order, reverse preorder with a prev pointer becomes a very stable in-place solution. Reading time: 12-15 min Tags: Hot100, binary tree, preorder, in-place, recursion SEO keywords: Flatten Binary Tree to Linked List, preorder, in-place, reverse preorder, LeetCode 114 Meta description: Learn LeetCode 114 from preorder order and reverse-preorder rewiring, with step-by-step derivation, engineering mappings, and runnable multi-language implementations. A — Algorithm Problem Restatement Given the root root of a binary tree, flatten the tree into a linked list in place. ...

Subtitle / Summary LeetCode 230 is not really about tree traversal mechanics alone. It is about turning BST order into a useful query. Once you see that the k-th smallest value is simply the k-th node visited in inorder traversal, the problem becomes a very stable counting task. Reading time: 11-14 min Tags: Hot100, binary tree, BST, inorder traversal, stack SEO keywords: Kth Smallest Element in a BST, BST, inorder traversal, stack, LeetCode 230 Meta description: Learn LeetCode 230 from BST inorder ordering, explicit-stack counting, and early-stop traversal, with runnable multi-language implementations. A — Algorithm Problem Restatement Given the root root of a binary search tree and an integer k, return the k-th smallest value in the tree. ...

Subtitle / Summary The most common confusion in LeetCode 543 is deciding what the recursive function should return. It should return height, not diameter. The diameter is updated globally at each node using leftHeight + rightHeight. Once that separation is clear, the problem becomes a clean introduction to tree DP. Reading time: 10-13 min Tags: Hot100, binary tree, tree DP, DFS, postorder SEO keywords: Diameter of Binary Tree, tree DP, height return, DFS, LeetCode 543 Meta description: Learn LeetCode 543 from the postorder height-return pattern, with step-by-step derivation, engineering analogies, and runnable multi-language solutions. A — Algorithm Problem Restatement Given the root root of a binary tree, return the diameter of the tree. ...

Subtitle / Summary The real difficulty of LeetCode 236 is not memorizing an LCA template. It is defining what each recursive call should return. Once that return value becomes stable, the whole problem collapses into a short and very clean postorder recursion. Reading time: 11-14 min Tags: Hot100, binary tree, LCA, DFS, postorder SEO keywords: Lowest Common Ancestor of a Binary Tree, LCA, postorder, DFS, LeetCode 236 Meta description: Learn LeetCode 236 from the recursive return-value semantics, with step-by-step derivation, engineering mappings, and runnable multi-language implementations. A — Algorithm Problem Restatement Given a binary tree, find the lowest common ancestor (LCA) of two given nodes p and q. ...

Subtitle / Summary The hardest part of LeetCode 98 is usually not recursion itself. It is realizing that checking a node against only its parent is not enough. A valid BST is constrained by all of its ancestors, so the stable solution is to pass a valid range downward. This guide explains that idea from scratch, then connects it to the equivalent inorder-sorted view. Reading time: 11-13 min Tags: Hot100, binary tree, BST, DFS, inorder traversal SEO keywords: Validate Binary Search Tree, BST, range constraints, inorder traversal, DFS, LeetCode 98 Meta description: Learn LeetCode 98 from the core invariant low < val < high, with step-by-step derivation, engineering mappings, and runnable multi-language implementations. A — Algorithm Problem Restatement Given the root root of a binary tree, determine whether it is a valid binary search tree (BST). ...

Subtitle / Summary Level order traversal is the entry point of the binary-tree BFS template. The real key is not merely “use a queue”, but “separate one level from the next correctly”. This ACERS guide explains the level-size pattern, the DFS depth-bucket alternative, and engineering situations where grouped-by-depth traversal is useful. Reading time: 10-12 min Tags: Hot100, binary tree, BFS, DFS, queue, level order traversal SEO keywords: Hot100, Binary Tree Level Order Traversal, BFS, queue, level order traversal, LeetCode 102 Meta description: A systematic guide to LeetCode 102 from level-by-level BFS to DFS depth buckets, with engineering scenarios and runnable multi-language implementations. Target Readers Hot100 learners who want to make the BFS tree template stable Developers who can traverse a tree but still mix up current-level and next-level boundaries Engineers who need to group tree-shaped data by depth for display or execution Background / Motivation LeetCode 102 is one of the most standard tree-BFS starter problems. ...