Linked Lists, Trees, Tries, Heaps

Learn — linked lists are pointers plus a dummy head, so draw before you type. Tree recursion returns exactly one thing; decide what before you write it. Tries are for prefixes. Heaps are for "top k" and streaming medians, where sorting is the wrong answer.

Do — problems 32–58. Implement LRU Cache without looking; it appears in real loops more than any other single design-flavoured question.

Check — you can explain why a doubly-linked list is required for O(1) LRU eviction; you can state what your tree recursion returns versus what it records; you can say why a min-heap answers a kth-largest query.

Ship — repo, same three-line comment format.

Diameter and Maximum Path Sum both turn on the same idea: the recursion returns one quantity to the parent and updates a different one globally. If that clicks, both are easy. If it does not, neither is.