142. Linked List Cycle II!

/**
 * 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) {
        // 1. check if there is a circle (slow and fast pointer)
        // 2. get the intersection: two pointer start from head and intersection
        ListNode startA = checkCycle(head);
        if(startA == null) return null;
        ListNode startB = head;
        while (startA!=startB){
            startA=startA.next;
            startB=startB.next;
        }
        return startA;
    }
    // if there is no cycle, return null
    // if there is a cycle, return the ListNode where slow and fast meet
    private ListNode checkCycle(ListNode head){
        ListNode slow = head;
        ListNode fast = head;
        
        // remember && not ||
        while(fast!=null && fast.next!=null){
            fast = fast.next.next;
            slow = slow.next;
            if(fast==slow) return slow;
        }
        return null;
    }
}