Java katas

Finger + verbal memory for the basics. Read the prompt, type the idiom from memory while saying what each line does, then check. Do a few every day so the fundamentals stop wobbling.

• Count frequencies into a MapMap
  Count how many times each word appears in a List<String>, into a Map<String,Integer>.
• Detect duplicates with a SetSet
  Iterate a list and detect duplicates using a Set.
• ArrayDeque as stack and queueDeque
  Use an ArrayDeque as both a stack (LIFO) and a queue (FIFO).
• Min-heap and max-heapHeap
  Create a min-heap and a max-heap of integers.
• Sort by field, then tie-breakComparator
  Sort a List<Person> by age ascending, then by name.
• Define an enumenum
  Define an offer-status enum.
• Define and use a recordrecord
  Define an immutable Point record and read a field.
• Avoid null with OptionalOptional
  Read a possibly-null user's name, defaulting to "anonymous".
• Validate argument and stateexceptions
  Reject a bad argument and an illegal state with the right exceptions.
• Stream: filter → map → collectstream
  From a List<Integer>, keep positives, double them, collect to a List.
• main() with a manual checktesting
  Write a main() that verifies add(2,3)==5 without a test framework.
• Two-pointer pair sumtwo-pointer
  Does a SORTED array have a pair summing to target? (two pointers)
• Sliding window (longest ≤ k)sliding-window
  Longest subarray with sum ≤ k (non-negative values).
• Binary searchbinary-search
  Find target's index in a sorted array, or -1.
• BFS with a visited setBFS
  BFS over an adjacency Map<Integer,List<Integer>> from start.
• Top-K frequent (count + heap)heap
  Return the k most frequent elements of an int[] (LC 347).
• Insert/Delete/GetRandom O(1)array+map
  A set with insert, remove, and getRandom all O(1) (LC 380).
• Merge overlapping intervalsintervals
  Merge overlapping intervals in an int[][].