You've worked through advanced problems.
BFS with column tracking handles vertical tree traversal. Use BFS, not DFS, for correct ordering.
Recursive LCA finds the lowest common ancestor by searching both subtrees.
Hash map + DFS clones graphs while handling cycles.
QuickSelect finds the kth element in average time.
Prefix sums + binary search handles weighted random selection in per pick.
Hash map + doubly linked list implements LRU Cache with operations.
Stack-based parsing evaluates expressions with operator precedence.