1) A lot of question around image compression and color reduction (One of my projects was on it.)
2) https://www.geeksforgeeks.org/detect-and-remove-loop-in-a-linked-list/amp/ (proof of hare tortoise algo was asked)
3) Detect if two line segments intersect each other.
4) A cylinder is rolled in the form of a sheet. Given two points find out the distance between them.