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Dynamic Programming
LIS, LCS, knapsack, chain multiply, and the rest — each with the recurrence spelled out and the traps flagged.
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Graphs
BFS/DFS, SCCs, Dijkstra, Bellman-Ford, MST — connectivity and shortest paths.
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Divide & Conquer
Master theorem, fast integer/matrix multiply, FFT, median-of-medians.
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NP-Completeness
Reductions, SAT, the standard NP-complete zoo, and how to write a reduction proof.