LeetCode 142. 环形链表 II

题目链接：LC 142. 环形链表Ⅱ

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2020.10.10第一次解答：

解题思路

1. 判断链表是否为环形链表。若是，用快慢指针返回环的长度（原理见一文搞定常见的链表问题），用于确定首个入环节点位置（末尾节点的 next 指针指向第一个入环节点）。若不是，返回0，代表链表非环形链表。
2. 若非环形链表，返回nullptr。若为环形链表，用双指针确定首个入环节点。

C++代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    int isCycle(ListNode *head) {
        ListNode *slow = head;
        ListNode *fast = head;
        while (fast) {
            fast = fast->next;
            if (fast) {
                fast = fast->next;
            }
            if (fast == slow) { //获取环长度
                int cnt = 0;
                while (fast) {
                    cnt++;
                    fast = fast->next;
                    if (fast) {
                        fast = fast->next;
                    }
                    if (fast == slow) {
                        return cnt;
                    }
                slow = slow->next;
                }
            }
            slow = slow->next;
        }
        return 0;
    }

    ListNode *detectCycle(ListNode *head) {
        int length = isCycle(head);
        if (length == 0)    return nullptr;
        ListNode *slow = head;
        ListNode *fast = head;
        for (int i = 0; i <= length; i++) {
            fast = fast->next;
        }
        while (fast != slow) {
            fast = fast->next;
            slow = slow->next;
        }
        return fast;
    }
};

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Java代码

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public int isCycle(ListNode head) {
        if (head == null || head.next == null)  return 0;
        ListNode slow = head;
        ListNode fast = head.next;
        while (slow != fast) {
            if (fast == null || fast.next == null)  return 0;
            slow = slow.next;
            fast = fast.next.next;
        }
        //此时已确定链表有环，进而获取环长度
        fast = fast.next;
        int cnt = 1;
        while (slow != fast) {
            fast = fast.next;
            cnt++;
        }
        return cnt;
    }

    public ListNode detectCycle(ListNode head) {
        int length = isCycle(head);
        if (length == 0)    return null;
        ListNode slow = head;
        ListNode fast = head;
        for (int i = 0; i < length; i++)    fast = fast.next;
        while (slow != fast) {
            slow = slow.next;
            fast = fast.next;
        }
        return fast;
    }
}