Linked List Cycle

Reorder List

Merge Two Sorted Lists

Detect a cycle from a linked list represented by next-node indices.

An array `nextIndices` describes a linked list whose head is node 0; each value is the next node index or `-1` for null. Return whether following next pointers enters a cycle.

## Example
```json
Input: {"nextIndices":[1,2,3,1]}
Output: true
```

## Constraints
- 0 <= nextIndices.length <= 10000
- Every entry is -1 or a valid index

## Follow-up
How does Floyd's slow/fast method translate to real node pointers?

The key pattern is: Floyd slow/fast pointers detect repeated traversal state. Identify the state and invariant before coding.

Recognize the Pattern

Advance one state transition at a time. Mark or update state before exploring dependent work.

Build the Algorithm

Test empty or minimal input, skewed shapes, duplicates where allowed, and impossible outcomes.