Two Pointers
01 / The shared mechanism
Two indices moving over the same sequence to maintain an invariant — the move rule changes per sub-pattern, but the mechanism is shared.
Pick a sub-pattern above. Each is independently linkable — start where the problem statement points.
↑ choose one— / When to use which
converging
Use when the array is sorted and the answer is a pair.
two-sum · palindrome · container
fast / slow
Use for in-place modification or cycle detection.
remove-dupes · linked-list cycles
sliding window
Use when the condition is about a contiguous chunk.
longest/shortest · at-most-k