51. Linked List Cycle Detection
Follow next links conceptually from the supplied head and decide whether this traversal eventually reaches a node already encountered. A cycle can begin at the head or later in the list.
Detect repeated node objects, not repeated values. Leave every input node and link unchanged.
Example 1
Input
N0(5) → N1(5) → null
Output
false
Explanation: Equal values at distinct identities do not form a cycle.
Example 2
Input
N0 → N1 → N2 → N3, with N3.next=N1
Output
true
Explanation: After three rounds slow and fast meet at N3. The meeting object need not be the cycle entry N1.
Example 3
Input
N0.next=N0
Output
true
Explanation: Both pointers move and meet after one round. Initial equality alone is not a test.
Constraints
0 <= nodes per input list <= 105-231 <= node.value <= 231 - 1- The next links may form one reachable cycle. Compare node identity, not repeated values.
Hints
Hint 1
A set of previously visited objects solves the task directly. What does a repeated identity prove?
Hint 2
Inside a cycle, what happens to the relative gap when one pointer moves twice as fast?
Hint 3
Move one and two edges under a null-safe guard, then compare references; null exhaustion means no cycle.
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Run your code to check the sample cases, or submit it to check the full validation suite.
Tail into a three-node cycle
[5,5,5,5]
1
true