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Diameter of Binary Tree
Diameter of Binary Tree teaches the post-order trick that powers half of all tree problems: each node returns its height while a side variable tracks the best answer seen. The path that matters rarely runs through the root, so a top-down count fails. Once the height-plus-side-effect pattern clicks, max-path-sum feels familiar.
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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