#0274

Linked List Cycle Detection

EasyDSA18 min10 XP

Problem

A linked list is represented by next_indexes, where next_indexes[i] is the next node index or -1. Starting from head, return True if the path enters a cycle.

Why This Matters

Cycle detection is the reason fast/slow pointers exist; it avoids storing every visited node.

Function Signature

def has_cycle(next_indexes, head):

Examples

Example 1

Inputnext_indexes = [1,2,0], head = 0

OutputTrue

0 -> 1 -> 2 -> 0 forms a cycle.

Constraints

Return the exact requested value.
Handle edge cases cleanly.
Use the intended DSA pattern when brute force would scale poorly.

CodePython

def has_cycle(next_indexes, head):
    """
    Input example: next_indexes = [1,2,0], head = 0
    Return: True
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return False

def has_cycle(next_indexes, head):
    """
    Input example: next_indexes = [1,2,0], head = 0
    Return: True
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return False

Visible browser tests run here when available.

Testcases2 visible / 3 hidden categories

Cycle exists

Input[1,2,0], head = 0

ExpectedTrue

Fast and slow eventually meet inside the loop.

No cycle

Input[1,2,-1], head = 0

ExpectedFalse

The path reaches the end marker.

Hidden Test Categories

head is -1cycle not starting at headsingle-node self cycle