core practice · coding problem
Number of Islands
LeetCode #200 — Number of Islands. Solve on LeetCode, then implement here if you want it in your drill queue. https://leetcode.com/problems/number-of-islands/
The mechanism
Most graph problems are one of a few templates: reachability/components (DFS/BFS), shortest path (BFS for unweighted, Dijkstra for weighted), ordering (topological sort), or connectivity (union-find). The skill is recognizing the template and modeling the input as nodes and edges.
Expected evidence
Pass all LeetCode test cases for this problem.
Try this drill in the practice workspace →
Hints when you need them
- Pattern hint: graphs.
- Start with brute force, then optimize time/space.
- Write down invariants before coding.
Watch for these mistakes
- No visited set — revisiting nodes and looping forever
- Using DFS for shortest path on an unweighted graph instead of BFS
- Not handling disconnected components