Array & String Find the first non-repeating character in a string. Rotate an array by K steps. Implement strstr() / substring search. Linked List Detect a cycle in a linked list. Reverse a linked list (iterative and recursive). Merge two sorted linked lists. Trees & Graphs Inorder / Preorder / Postorder traversal. Check if a binary tree is balanced. Find the shortest path in a graph (BFS/DFS, Dijkstra).