Q-14 Linked List Cycle II - LeetCode

/**
 * 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) {
        if(head == null || head.next == null)   return null;
        
        ListNode fast = head;
        ListNode slow = head;

        //checking if cycle is present
        while(fast!= null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;

            if(fast == slow) break;
        }
        
        if(fast != slow) return null;

//cycle is present now fast and slow will be at meeting point 
// now we will keep slow at head again and move them at together at one unit speed only to find the
// the starting cycle node

        slow = head;
        //fast = fast;

        while(fast != slow){
            slow = slow.next;
            fast = fast.next;
        }

//now they are at the starting cyclic node 
        return slow;

        //we can also find A , B+C , m' using the equation now

    }
}