You learned how to count arrangements where order matters.
Key formulas: n! for full permutations, P(n,r) = n!/(n-r)! for partial, n^r with repetition, n!/(n₁! × ... × nₖ!) for multisets, (n-1)! for circular.
You solved permutation generation problems using backtracking, handled duplicates with sorting and pruning, and saw applications in passwords and DNA.
Next section: combinations, where order does not matter.