Basics: traversal, insertion, deletion • Techniques: sliding window, two pointers • Common problems: • Two Sum • Longest Substring Without Repeating Characters • Maximum Subarray (Kadane’s Algorithm) 2. Linked Lists • Types: singly, doubly, circular • Operations: reversal, merge, detect cycles • Common problems: • Reverse a Linked List • Detect Cycle (Floyd’s Cycle Detection) • Merge Two Sorted Lists 3. Stacks & Queues • Applications: expression evaluation, backtracking, BFS • Variants: monotonic stack, deque • Common problems: • Valid Parentheses • Min Stack • Sliding Window Maximum 4. Hashing (Hash Maps & Sets)