Admissibility
h(n) ≤ actual distance to target. Guarantees optimal path found. Zero heuristic → A* = Dijkstra.
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Consistency
h(u) ≤ w(u,v) + h(v). Stronger than admissibility. Ensures node closed only once (no re-opening).
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Common heuristics
Grid: Manhattan, Euclidean, Chebyshev. Road networks: straight-line distance. Puzzles: pattern databases.