142. Linked List Cycle II

寻找cycle的开始节点。相比快慢指针另外多加了一个指针entry，用来从头寻找cycle入口

public ListNode detectCycle(ListNode head) {
    ListNode entry = head,
             fast = head,
             slow = head;
    while (fast != null && fast.next != null) {
        fast = fast.next.next;
        slow = slow.next;
        if (fast == slow) {
            while (entry != slow) {
                entry = entry.next;
                slow = slow.next;
            }
            return entry;
        }
    }
    return null;
}