https://oj.leetcode.com/problems/linked-list-cycle-ii/

Linked List Cycle II.java

/**
 * 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 first = head;
        ListNode second = head;
        boolean flag = false;
        while ((second.next != null) && (second.next.next != null)){
            first = first.next;
            second = second.next.next;
            if (first == second) {
                flag = true;
                break;
            }
        }
        if (flag == false) {
            return null;
        } else {
            ListNode circleStart = second;
            int circleLength = 1;
            while (second.next != circleStart){
                circleLength++;
                second = second.next;
            }
            first = head;
            do{
                second = first;
                for (int i = 0; i < circleLength; i++){
                    second = second.next;
                }
                if (second == first) break;
                first = first.next;
            }while(true);
            return first;
        }
    }
}