DSA Problem 1: Detect cycle in undirected graph using DFS; track parent & recursion stack. O(V+E). ​ DSA Problem 2: Merge k sorted arrays; use min-heap. O(NK log K). ​ Redux: Store holds state, actions trigger reducers for immutable updates via dispatch. ​ HTTP 200: OK, success. 404: Not Found. 500: Server Error. ​ JS Closures: Function retains outer scope access. Hoisting: var declared top-level.