dsa · easy

Linked List Cycle II (start)

You are given the head of a singly linked list that may contain a cycle. Return the **node** where the cycle begins, or null if there is none. The judge prints that node's 0-based index, or -1.

Arguments

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

Only head. Return the node, not an index. Walk .next.

Example

3 → 2 → 0 → -4 with the tail linked back to 2 → return that node (judge prints 1). No cycle → -1.

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.

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:
1

Example 2

Input:
[1,2]
-1

Expected:
-1

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