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Clone Graph
Clone Graph is the canonical visited-map problem: as you DFS, a map from old node to new node both stops infinite loops and wires up the copied edges. Skip the map and a cycle sends you spinning forever. Once you see why one structure does double duty, most graph-copy problems collapse into it.
Learn this pattern
Graph Interview Questions
BFS explores by layers using a queue; DFS explores deeply using a stack or recursion -- the same traversal with a different container. Choose BFS for shortest-path in unweighted graphs and "minimum steps," DFS for counting components, islands, and tree recursion.
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