DSA Quest

Key market

HashSet for membership, Dictionary for counting and complement lookups, canonical keys for grouping, prefix-sum counts, and prefix×suffix products. Contains Duplicate, Two Sum, Group Anagrams, Subarray Sum Equals K, Product Except Self.

Twin-lantern bridge

Converge two indices toward each other, using sorted structure to prove a move is safe before making it. Container With Most Water, 3Sum, Valid Palindrome (skip junk, compare ends).

Observatory train

Grow a window while a constraint holds, shrink it the moment the constraint breaks. Longest Substring Without Repeating Characters, Minimum Window Substring, fixed-k sums, and the rate limiter every API gateway runs.

Running-sum ridge

Pay once for a running total, then answer range sums, balance points, and modular subarray checks in linear time. Range Sum Query, Pivot Index, Continuous Subarray Sum. (Subarray Sum Equals K also appears under Arrays & Hashing as the hash hybrid.)

Crystal cave

Every comparison eliminates half of everything that could still be the answer. Works on sorted arrays, answer ranges like Koko Eating Bananas, rotated sorted search, and finding the rotation minimum.

Plate tower temple

Use last-in-first-out state for nesting, undo, traversal, next greater element, daily temperatures, and nested decode-string expansion.

Node vine forest

Follow references, preserve links while rewiring, detect cycles, merge sorted lists, and remove the nth node from the end with a pointer gap.

Calendar ridge

Sort ranges, merge overlaps, insert into a sorted day, and size rooms by peak concurrency. Merge Intervals, Insert Interval, Meeting Rooms II.

Call-stack spire

Phase 4: call stack frames, base case + progress, and Climbing Stairs recurrence. Foundation for Trees and Graphs.

Canopy trail

Binary nodes, DFS preorder/inorder/postorder, and maximum depth. Builds on recursion’s null base case and call stack.

Node web

Adjacency lists, BFS hop layers (queue + visited), and DFS with a visited set for cycles. Builds toward topo sort and weighted shortest paths later.

Peak heap

Binary heap property in an array, push/pop with sift up/down, and top-K with a size-K min-heap. Priority queues for schedulers and leaderboards.

Memo maze

Phase 5 DP: stairs, house robber I/II, unique paths I/II, coin change, LIS, edit distance, knapsack, LCS, min path, partition, target sum, decode ways, max product.