Flattening

The 'Flattening a Linked List' problem is an advanced challenge where you are given a linked list where each node represents a head of another, separate linked list. Each of these 'down' lists is sorted, and the main 'right' list is also sorted by the head nodes. The task is to flatten the entire structure into a single sorted list. For example, you might have a main list 5 -> 10 -> 19 -> 28, where the node '5' also has a 'down' list 7 -> 8 -> 30, and '10' has a 'down' list '20', etc. The final output should be a single list: 5 -> 7 -> 8 -> 10 -> 19 -> 20 -> 28 -> 30. A very effective approach to solve this is using recursion and a merge operation. The idea is to treat the problem as merging two sorted lists at a time. We can define a recursive function that takes the head of the list as input. The base case is when the head is null or its `next` (right) pointer is null. In the recursive step, we first flatten the rest of the list starting from `head->next`. This gives us a single, sorted, flattened list. Now, we just need to merge the current node's 'down' list with this flattened 'rest of the list'. The standard `mergeTwoSortedLists` algorithm can be used for this merge step. This divide-and-conquer approach elegantly breaks the problem down into a series of merge operations.