Linked List — Where Does the Cycle Start?

A singly linked list normally ends in null. A corrupted one has its last
node pointing back at an earlier node, so walking it never ends.

solve(values, pos) builds a list from values. If pos >= 0, the tail's
next is wired back to the node at index pos; if pos === -1 the list is
left intact. Return the index of the node where the cycle begins, or -1
when the list has no cycle.

Requirement: use O(1) extra space. A Set of visited nodes is the
obvious answer and is not accepted here — use Floyd's tortoise and hare.

Examples

solve([3, 2, 0, -4], 1)  → 1    // tail points back at the node holding 2
solve([1, 2],        0)  → 0    // tail points back at the head
solve([1],          -1)  → -1   // no cycle

Constraints

• values may be empty (then the answer is -1).
• Values may repeat, so compare nodes, never values.