【剑指offer】链表中环的入口结点

public class  {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}

public ListNode EntryNodeOfLoop(ListNode pHead) {
    ListNode node = meetingNode(pHead);

    if (node == null)
        return null;

    int count = countNodes(node);

    return findEntry(pHead, count);
}

private ListNode meetingNode(ListNode head) {
    if (head == null)
        return null;

    ListNode slow = head;
    ListNode fast = head.next;

    while (fast != null) {
        if (fast == slow)
            return slow;

        if (fast.next == null)
            return null;

        slow = slow.next;
        fast = fast.next.next;
    }

    return null;
}

private int countNodes(ListNode node) {
    ListNode current = node.next;
    int count = 1;

    while (current != node) {
        count++;
        current = current.next;
    }

    return count;
}

private ListNode findEntry(ListNode head, int count) {
    ListNode slow = head;
    ListNode fast = head;

    for (int i = 0; i < count; i++)
        fast = fast.next;

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

    return slow;
}