Lesson 12
Lower/upper bound, binary search on answer, mảng xoay.
Lesson 13
Đối đầu, cùng chiều, fast-slow (cycle detection).
Lesson 14
Fixed/variable window, tối ưu O(n²)→O(n).
Lesson 15
Prefix sum 1D/2D, difference array, range query.
Lesson 16
Frequency map, dedup, two-sum pattern (link DataStructures/hash).
Lesson 17
Paradigm tổng quát: closest pair, count inversions, quickselect.
Lesson 18
Khung backtracking, permutation/combination/subset, pruning.