Reverse a Linked List / Detect Cycle

Problem Reverse a singly linked list, and separately detect whether a list contains a cycle.

Input / Output

• Input: head of a singly linked list.
• Output: the new head after reversal; and a boolean for whether a cycle exists.

Constraints

• 0 <= nodes <= 10^4; do both in O(1) extra space.

Example

• 1→2→3 reverses to 3→2→1. A list whose tail points back to node 2 has a cycle.