Solve linked-list and top-K algorithm tasks

##### Question Given a singly linked list, return the k-th node counted from the end. Given an integer array, return the top k most frequent numbers. Given a list of points on the 2-D plane, return the k points closest to the origin (0, 0). Given a string where adjacent identical characters "collide" and disappear repeatedly (e.g., "abbba" → "aa" → ""), output the final string after all collisions are resolved. https://leetcode.com/problems/remove-nth-node-from-end-of-list/description/ https://leetcode.com/problems/top-k-frequent-elements/description/ https://leetcode.com/problems/k-closest-points-to-origin/description/ https://leetcode.com/problems/remove-all-adjacent-duplicates-in-string-ii/description/