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Invert Binary Tree
This is really about exploring a state space without revisiting the same work. It's a great warm-up that builds traversal modeling without a ton of moving parts. Be clear about what a state is; that's the difference between BFS/DFS working or blowing up.
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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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Graph Interview Questions gives you a useful next rep while you are still on your phone.