#1255)

* Refactor RedBlackTree: encapsulate Node fields, standardize naming, and expand tests (#1255)

- Encapsulate Node fields (color, value, left, right, parent) with private access and provide getters/setters
- Standardize variable naming conventions (camelCase)
- Unified search logic in contains()
- Remove unused methods: updateParentChildLink, findMin, findMax, swapColors
- Expand RedBlackTreeTest to include comprehensive invariant verification:
  - Root and NIL node color checks
  - Red-node property (red nodes must have black children)
  - Black-height property (consistent black nodes on all paths to NIL)
- Integrated invariant verification into randomized stress tests

* Simplify RedBlackTree implementation: consolidate symmetric cases and rotations

* Use more descriptive variable names in RedBlackTree for better readability

* Further simplify RedBlackTree: use public Node fields to match project style

- Make N field private in both FenwickTree classes
- Add proper import for java.util.Arrays, fix misleading constructor comment
- Add input validation: negative size, out-of-range indices in sum()
- Add size() method to FenwickTreeRangeQueryPointUpdate
- Remove commented-out debug prints and dead assertions in tests
- Expand RangeQueryPointUpdate tests from 7 to 14
- Expand RangeUpdatePointQuery tests from 6 to 10

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

…age (#1258)

* Cleanup and simplify hashtable implementations and improve test coverage

* Restore educational comments and update .gemini.md

…1257) (#1261)

- Standardize variable naming to camelCase (removedData, elemI, elemJ)
- Add null checks in constructors and add/contains/remove methods
- Expand test suite to cover:
  - Empty heap peek/poll behavior
  - Null input handling for all public methods
  - Constructor validation for arrays and collections with null elements
  - toString representation
- Preserve all educational comments

* Cleanup DoublyLinkedList and expand test coverage

- Add proper import for java.util.Iterator instead of inline qualified names
- Change addAt to throw IndexOutOfBoundsException instead of checked Exception
- Remove pointless local variable assignments in clear() and remove(Node)
- Move trav declaration to point of use in remove(Object)
- Replace inline java.util imports with proper imports in tests
- Expand test suite from 18 to 30 tests covering contains, indexOf,
  null handling, invalid indices, iterator, and forEach

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Disallow null elements in DoublyLinkedList

- Reject null in addFirst/addLast with IllegalArgumentException
- Simplify remove(Object) and indexOf(Object) by removing null branches
- Replace null-related tests with testNullRejection
- Remove null entries from randomized test data generators

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>