Flatten a Multilevel Doubly Linked List (Stack)

I would approach flattening this multilevel doubly linked list using an iterative method with an explicit stack to ensure clarity and avoid recursion depth issues. First, I initialize a stack and start traversing from the head. As I iterate through each node, if I encounter a node with a child pointer, I save the current 'next' node by pushing it onto the stack, then redirect the current node's 'next' to point to its 'child'. Crucially, I must update the 'prev' pointer of the new next node to point back to the current node and clear the child pointer to zero. Once I reach the end of the current path, if the stack is not empty, I pop the saved 'next' node and continue traversal. This ensures that any deferred branches are processed immediately after their parent branch is fully flattened. For example, if we have A -> B(child C) -> D, when at B, I push D, link B to C, and later resume with D. This approach guarantees O(N) time complexity as each node is visited exactly once, and O(N) space for the stack in the worst case where all nodes form a single vertical chain. This solution aligns well with IBM's focus on robust, memory-efficient algorithms for enterprise-grade systems.