LinkedListLoopDetectAndStartPoint

/**
 * 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 ) return null;
        ListNode slow = head , fast =head;
        boolean loopExist =false;
        
        while(fast.next != null && fast.next.next != null){
             fast = fast.next.next;
             slow = slow.next;
          if(fast == slow ){
              loopExist = true;
              break;
          }
        }
        
        if(loopExist){
            
            slow = head;
            
            while(slow != fast){
                
                slow= slow.next;
                fast = fast.next;
            }
            return slow;
        }
       return null; 
    }
}