Added 2 leetcode hard problems

Author: Sunil4510

assignments/15-linkedlist.md

@@ -36,3 +36,5 @@
 - [LFU Cache](https://leetcode.com/problems/lfu-cache/) `leetcode` `Google`
 - [Merge k Sorted Lists](https://leetcode.com/problems/merge-k-sorted-lists/) `leetcode`
 - [Clone a linked list with next and random pointer](https://www.geeksforgeeks.org/clone-linked-list-next-random-pointer-o1-space/) `GFG` `Google` `Flipkart`
+- [All O'one Data Structure](https://leetcode.com/problems/all-oone-data-structure/) `leetcode`
+- [Design Skiplist](https://leetcode.com/problems/design-skiplist/) `leetcode`