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Count Good Nodes in Binary Tree
Count Good Nodes in Binary Tree is a traversal problem: the hard part is modeling the state you visit. Interviewers love it because it reveals your traversal modeling and your edge-case discipline. Once your traversal structure is solid, you can reuse it across dozens of problems.
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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