dsa · easy

Linked List Cycle

You are given the head of a singly linked list. Each node has val and next. Return whether following next ever comes back to a node you already visited.

Arguments

• head — a ListNode (val, next); None if the list is empty

Only head. Walk .next. Empty list → false.

Example

3 → 2 → 0 → -4 with the tail linked back to 2:

`` 3 → 2 → 0 → -4 ↑ | └─────────┘ ``

Samples print **node values**, then how the judge wired the tail (an index, or -1 for no cycle). That second number is **not** a parameter. If it were, return pos != -1 would solve the problem.

Constraints

0 <= number of nodes <= 4*10^4 Hidden tests include near-max size for this bound; a slower-than-intended solution TLEs.

Examples

Example 1

Input:
[3,2,0,-4]
1

Expected:
true

Example 2

Input:
[1,2]
-1

Expected:
false

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