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Nodes, Edges, and the Visited Set
Graphs are just nodes plus edges — learn the adjacency list, the undirected two-way rule, and why cycles make a visited set mandatory.
Complete codeMultiple choice - 2
Find Center of Star Graph
The center touches every edge — so two edges are all the evidence you need. Structure guarantees can turn O(n) into O(1).
Complete codeMultiple choiceTap - 3
Find the Town Judge
Trust pairs are directed edges — the judge is pure degree bookkeeping: in-degree n - 1, out-degree 0, one score array.
Complete codeDebugMultiple choiceTrace - 4
Find if Path Exists in Graph
Your first real traversal: turn the edge list into an adjacency list (both directions!), then BFS outward from the source with a visited set until the destination appears — or the frontier runs dry.
Complete codeMultiple choiceTrace - 5
Keys and Rooms
Reachability in disguise: rooms are nodes and keys are directed edges. Start in room 0, collect keys as you go, and succeed only if visited ends up covering every room.
Complete codeDebugMultiple choiceTrace - 6
All Paths From Source to Target
From 'can I get there?' to 'every way to get there': DFS carries the current path, records a COPY at the target, and backtracks on the way out. No visited set — it would erase answers.
Complete codeDebugMultiple choiceTrace - 7
Graph Foundations CheckpointCheckpoint
No new concepts — transfer the island's graph moves to unseen problems: counting connected groups from a matrix, zero in-degree reasoning, tree-shaped edge lists, and celebrity-style degree bookkeeping.
Complete codeCode orderMultiple choiceTrace - 8
Graph Foundations ReviewReview
Mixed recap of the island: building undirected adjacency both ways, why cycles demand a visited set, when counting degrees beats walking, reachability versus path enumeration, and where O(V + E) comes from.
Complete codeMultiple choiceTrace - 9
Boss: Clone GraphBoss
The island's final test: deep-copy a connected undirected graph — every node AND every edge rebuilt as new objects. The trap: a visited SET can say 'been here' but cannot hand back the clone you built; an old-to-new HashMap does both jobs at once.
Complete codeDebugCode orderMultiple choiceTrace
Trees & Graphs is part of AlgoFox Pro
You can read the whole course outline here. The exercises are in the app, along with your streak, your mistake review, and the progress that carries across your devices.
Pro starts with a 3 day free trial. The app store decides who is eligible for an introductory offer, and a lapsed subscription does not get another one.
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