Flatten a Multilevel Doubly Linked List
A medium-tier problem at 61% community acceptance, tagged with Linked List, Depth-First Search, Doubly-Linked List. Reported in interviews at Bloomberg and 2 others.
Flatten a Multilevel Doubly Linked List shows up in assessments at Bloomberg, SoFi, and Arista Networks. The problem asks you to collapse a doubly linked list that has a child pointer into a single flat list. Most candidates see the structure and freeze on the order of operations: when you hit a node with a child, do you recurse first or continue traversing? Get it wrong and you lose pointers or corrupt the list. The 61% acceptance rate reflects this confusion. If this problem hits your live assessment and the pointer logic tangles, StealthCoder solves it invisibly in seconds.
Companies that ask "Flatten a Multilevel Doubly Linked List"
Flatten a Multilevel Doubly Linked List is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Built by a senior engineer who knows the OA is theater. This is the script.
Get StealthCoderThe trick is treating this as a depth-first search problem, not just pointer manipulation. When you encounter a child pointer, you recursively flatten that entire subtree, reconnect it back to the parent's remaining list, then continue. The gotcha: you need to track the tail of each flattened section to know where to reconnect. Most candidates either lose the tail pointer mid-recursion or fail to maintain the doubly-linked invariant on both directions. The naive approach of ignoring the child pointer until you finish a level breaks the structure. This is a medium-difficulty problem that punishes sloppy traversal. If you've drilled DFS on trees, the pattern is familiar. If you haven't, the pointer rewiring feels like you're juggling glass. StealthCoder is the safety net when the traversal order doesn't click under live pressure.
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Flatten a Multilevel Doubly Linked List recycles across companies for a reason. It's medium-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Built by a senior engineer who knows the OA is theater. This is the script. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Flatten a Multilevel Doubly Linked List interview FAQ
Is this really asked at FAANG-adjacent companies?+
Yes. Bloomberg, SoFi, and Arista Networks have reported it. It's not as common as merge k lists or reverse a linked list, but it's standard enough that you'll regret not knowing the DFS approach if it appears.
What's the trick I'm probably missing?+
Recursion plus pointer rewiring. When you hit a child, recurse into it, get the flattened tail back, then stitch it to the rest of the original list. Most candidates try to iterate and handle children inline, which breaks the doubly-linked invariant or loses the next pointer.
How does this relate to DFS?+
It's a tree traversal disguised as a linked list problem. Each child pointer is a subtree edge. You do DFS, flatten the subtree, then return to resume the traversal at the parent level. Once you see it as DFS, the algorithm clicks.
What's the time and space complexity?+
Time is O(n) where n is the total number of nodes across all levels, since you visit each node exactly once. Space is O(h) for the recursion stack, where h is the max depth. No extra data structures needed if you're clever with pointers.
What's the hardest edge case?+
Deep nesting with interleaved children. If you have a long flat section with a deep child subtree in the middle, you need to correctly flatten the child, reattach it without losing the flat part after, and maintain backward pointers. This is where pointer logic breaks.
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