（转载）链表环中的入口点 编程之美 leecode 学习

http://www.cnblogs.com/hiddenfox/p/3408931.html 说的很细

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {

    public static ListNode detectCycle(ListNode head) {
    ListNode slow = head;
    ListNode fast = head;

    while (true) {
        if (fast == null || fast.next == null) {
            return null;
        }
        slow = slow.next;
        fast = fast.next.next;
        if (fast == slow) {
            break;
        }
    }

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