Linked List Duplicate Elements | Dataford Interview Questions

Calsoft integration workflows may receive linked lists containing repeated identifiers from upstream processing. Given the head of a singly linked list, return every value that appears more than once, preserving the order in which each duplicate is first identified.

Formal Specification

The input is head, a reference to the first ListNode. Each node has an integer val field and a next field containing the next node or None. Return a Python list containing each duplicated value exactly once. If no value appears more than once, return an empty list.

For the examples and test cases, a linked list is represented by an array of node values. The evaluator converts that array into the corresponding ListNode chain before calling the function.

Constraints

0 <= n <= 10^5
-10^9 <= node.val <= 10^9
The input is a singly linked list
Duplicate values must appear once in first-duplicate order

Function Signature

def find_duplicates(head):

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