Q18. Reverse a linked list. ➡️ Iterative method: maintain prev, curr, next pointers. Update links until curr = NULL. Return prev as new head. Q19. Detect a cycle in linked list. ➡️ Use Floyd’s Cycle Detection (slow & fast pointer). If they meet, cycle exists. Q20. Longest substring without repeating characters. ➡️ Use sliding window + hash set. Move right pointer, if char repeats, move left pointer. Keep track of max length. Q21. Binary search. ➡️ Divide array in half, check mid, narrow search space to left/right depending on target. O(log n). Q22. N-Queens. ➡️ Use backtracking. Place queen row by row, check safety (column, diag), recurse. If placement fails, backtrack.