LeetCode-142-Linked-List-Cycle-II.java

/*
LeetCode:  https://leetcode.com/problems/linked-list-cycle-ii/
LintCode:  http://www.lintcode.com/problem/linked-list-cycle-ii/
JiuZhang:  http://www.jiuzhang.com/solutions/linked-list-cycle-ii/
ProgramCreek:  not find
http://fisherlei.blogspot.com/2013/11/leetcode-linked-list-cycle-ii-solution.html

Analysis:  
1.Two Pointers

2.Two Pointers


*/
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    // 1.Two Pointers
    public ListNode detectCycle(ListNode head) {
        if (head == null || head.next == null) return null;     // there is no cycle
        
        ListNode slow = head, fast = head.next;
        while (slow != fast) {
            if (fast == null || fast.next == null) return null; // means there is no cycle
            slow = slow.next;
            fast = fast.next.next;
        }
        
        fast = head;
        slow = slow.next;       // we must let slow starts from the first one in the next round
        while (fast != slow) {
            slow = slow.next;
            fast = fast.next;
        }
        
        return slow;
    }
    
    // 2.Two Pointers
    public ListNode detectCycle(ListNode head) {
        if (head == null) return null;
        
        ListNode slow = head, fast = head;
        
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            
            if (slow == fast) {
                System.out.println(slow.val);
                break;
            }
        }
        
        if (fast.next == null || fast.next.next == null) return null;
        
        while (head != slow) {
            head = head.next;
            slow = slow.next;
        }
        
        return head;
    }
    
}