鏈表中環的入口節點

給一個鏈表，若其中包含環，請找出該鏈表的環的入口結點，不然，輸出null。

思路：

public ListNode EntryNodeOfLoop(ListNode pHead) {
    if (pHead == null || pHead.next == null)
        return null;
    ListNode slow = pHead, fast = pHead;
    do {
        fast = fast.next.next;
        slow = slow.next;
    } while (slow != fast);
    fast = pHead;
    while (slow != fast) {
        slow = slow.next;
        fast = fast.next;
    }
    return slow;
}