Each topic is a deterministic browser animation: source code, memory cells, CPU stage, and call stack stay connected in one repeatable trace.
The catalog maps the full interview pattern set. Every topic now opens as an execution trace.
Why haystack's i does not move back when matching fails.
Why left, right, and mid updates do not skip the answer.
Why the left pointer only moves when the window becomes invalid.
How a hash map and doubly linked list make get and put O(1).
Why slow and fast pointers can overwrite safely without extra memory.
How maps turn search, duplicate checks, and frequency comparison into O(n).
How sorting keys, start markers, and candidate compression change the problem.
Why moving one side can safely discard many brute-force pairs.
Why interval problems usually start with sorting and one active range.
How prev, cur, next, dummy, and tail make linked-list code stable.
Why moving first and comparing later avoids false cycle hits.
How LIFO and FIFO constraints shape data-structure problems.
Why nested structure and reverse polish notation fit a stack.
Why a size-k heap keeps only the useful candidates.
How scanning, slicing, and prefix checks avoid off-by-one errors.
How a recursive function defines the work for one subtree.
Why queue length freezes one level before children are appended.
How visit timing changes the meaning of the traversal.
How BST order turns tree problems into range and inorder problems.
How path records, recursive gains, and global best differ.
How root position, inorder split, and null markers rebuild structure.
How path, choices, end condition, and undo step generate the search tree.
How visited state and four directions turn a matrix into a graph.
How adjacency lists, indegree, and frontier expansion model graph problems.
How parent pointers compress connectivity questions.
How prefix trees and segmentation DP solve word lookup problems.
How state definition, initialization, transition, and answer extraction fit together.
How to prove a local choice does not hurt the global answer.
Why the stack stores unresolved indices instead of values only.
How subarray sum problems become differences between two prefixes.
How a monotonic feasibility function turns optimization into search.
How multiple structures combine to satisfy every operation contract.
How row, column, and layer boundaries control matrix traversal and mutation.
How ordering, partitioning, and merge boundaries support many interview solutions.
How binary representation, carry, modulo, and powers explain compact solutions.
How interval updates become boundary events and prefix accumulation.
How priority queues and edge ordering solve weighted shortest path and connection problems.
How Fenwick trees, segment trees, and ordered sets keep dynamic ranges queryable.
How hashes, palindromic radii, and Z boxes reuse previous comparisons.