Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Note: Do not modify the linked list.

Follow up:
Can you solve it without using extra space?

Detailed mathmetic explanation

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/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        ListNode fast = head, slow = head;
        boolean isCycle = false;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) {
                isCycle = true;
                break;
            }
        }
        
        // # of fast = 2 * # of slow = # of meeting node + circle = # of slow + circle
        // => # of slow = circle
        // distance from head to the circle entry = distance from meeting node to circle entry
        if (!isCycle) return null;
        while (head != slow) {
            head = head.next;
            slow = slow.next;
        }
        return head;
    }
}