Linked List (Medium) Remove Nth Node From End of List Pattern: Two pointers Learn: Fast and slow pointers Dummy node Edge cases Reorder List Pattern: Find middle Reverse second half Merge two lists One of the most important linked list questions. Sort List Pattern: Merge Sort on Linked List Very common interview question. Swap Nodes in Pairs Pattern: Pointer manipulation Dummy node Odd Even Linked List Pattern: Rearranging pointers Rotate List Pattern: Length calculation Circular linked list Breaking at correct position Sliding Window (Medium) Longest Repeating Character Replacement Pattern: Variable-size window Frequency array Permutation in String Pattern: Fixed-size window Character count Maximum Erasure Value Pattern: Sliding window + HashSet Fruit Into Baskets Pattern: At most K distinct Longest Substring Without Repeating Characters Pattern: Classic sliding window Minimum Size Subarray Sum Pattern: Variable window Shrink while valid Trees (Medium) Binary Tree Right Side View Pattern: BFS level order DFS alternative Validate Binary Search Tree Pattern: Range checking Inorder traversal Lowest Common Ancestor of a Binary Tree Pattern: DFS recursion Path Sum II Pattern: Backtracking on trees Construct Binary Tree from Preorder and Inorder Traversal Pattern: Divide and conquer Hash map Kth Smallest Element in a BST Pattern: Inorder traversal BST property