DSA interview question

Insert a Node Before the Head of a Doubly Linked List

Easy · Data structures · Topics: linkedlist, doubly linkedlist

The problem

The browser from the previous problem keeps its history as a doubly linked list, and opening a page from a "recently closed" menu inserts it at the front. In a doubly linked list, inserting before the head means wiring two links instead of one: the new node's next points to the old head, and the old head's prev points back to the new node. Forgetting the second link is the classic bug, and it only shows up when you traverse backwards. The same two-sided wiring applies when inserting at any position.

Example: 2 <-> 3, insert 1 before the head → 1 <-> 2 <-> 3.

Function signature

DNode* insertBeforeHead(DNode* head, int x);             // returns the new head
DNode* insertAtPosition(DNode* head, int pos, int x);    // 1-indexed, 1 <= pos <= length + 1

Test cases

Input	Forward result	Explanation
`insertBeforeHead(2 <-> 3, 1)`	`1 <-> 2 <-> 3`	Backward: `3 <-> 2 <-> 1`
`insertBeforeHead(empty, 5)`	`5`
`insertBeforeHead(7, 7)`	`7 <-> 7`
`insertAtPosition(1 <-> 2 <-> 4, 3, 3)`	`1 <-> 2 <-> 3 <-> 4`	Four links change
`insertAtPosition(1 <-> 2, 1, 0)`	`0 <-> 1 <-> 2`	Same as inserting before the head
`insertAtPosition(1 <-> 2, 3, 9)`	`1 <-> 2 <-> 9`	At the tail

(Confirm: after every insertion, the backward traversal must be the exact reverse of the forward one.)

How interviewers phrase it

"Insert a node at the beginning of a doubly linked list."
"Insert a node after a given position in a doubly linked list" (GeeksforGeeks).
Part of implementing a deque or an LRU cache.

Clarifying questions to ask

Is the position 0- or 1-indexed?
Should the function return the new head?
Is there a tail pointer to maintain as well?

Follow-ups to expect

"How many pointers change when inserting in the middle?"
"What happens if you forget oldHead->prev = newNode?"
"How do you verify a doubly linked list?"
"Complexity?"

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