Find existing value
Input
head = build_linked_list([4, 5, 6]) find_node(head, 5)
Output
1
5 is at index 1 (0-based).
Full lesson preview
Find a node by value
Locate the first node that contains a given value and return its index.
Python practice8 minLinked ListsBeginnerLast updated March 27, 2026
Problem statement
Given the head of a singly linked list and an integer target value, return the index (0-based) of the first node whose value equals the target. If the target doesn't appear in the list, return -1. If the list is empty, return -1.
Task
Implement a function that returns the 0-based index of the first node containing the target value, or -1 if not found.
Examples
Input format
Two arguments: head (ListNode or None), value (int).
Output format
An integer index (0-based) or -1.
Constraints
- Node values and target are integers.
- List length up to 10^5; run in O(n) time and O(1) extra space.
Samples
Sample 1
Input
build_linked_list([7,7,8]), 8
Output
2
The target 8 appears at index 2.