Master backtracking — subsets, combination sum, and permutations
What backtracking is, the choose/don't choose pattern, and when to use it.
Build every binary string of length n — two choices at each step.
Build every sequence of length k using numbers 1 to n, with repetition allowed.
Return all possible subsets of unique integers — the classic backtracking problem.
All unique combinations that sum to target — numbers can be reused.
Return all possible permutations of unique integers — each number used once.