Coding Challenge Solutions

Combine a doubly linked list for ordering with a hash map for O(1) lookups. Move accessed nodes to the front; evict from the tail.

Apply Kadane's algorithm: maintain a running sum and reset it when it drops below zero. Track the global maximum throughout.

The number of ways to reach step n is the sum of ways to reach steps n-1 and n-2 — essentially the Fibonacci sequence.

Swap the left and right children at every node recursively. Base case: if the node is null, return null.