LeetCode 刷题【142. 环形链表 II】

142. 环形链表 II

自己做

解：三指针

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        if(head == null || head.next == null)           //必定无环
            return null;
        
        ListNode p = head;
        ListNode q = head.next;
        ListNode r = head.next;

        while(r != null){
            r = r.next;                                 //快指针
            if(r != null)
                r = r.next;

            q = q.next;                                 //慢指针

            if(q == p)                                  //遇到入口
                return p;

            if(r == q)                                  //快指针追上了慢指针（转了一圈）
                p = p.next;

        }   

        //无环
        return null;     
    }
}

看题解

public class Solution {
    public ListNode detectCycle(ListNode head) {
        if (head == null) {
            return null;
        }
        ListNode slow = head, fast = head;
        while (fast != null) {
            slow = slow.next;
            if (fast.next != null) {
                fast = fast.next.next;
            } else {
                return null;
            }
            if (fast == slow) {
                ListNode ptr = head;
                while (ptr != slow) {
                    ptr = ptr.next;
                    slow = slow.next;
                }
                return ptr;
            }
        }
        return null;
    }
}

作者：力扣官方题解
链接：https://leetcode.cn/problems/linked-list-cycle-ii/solutions/441131/huan-xing-lian-biao-ii-by-leetcode-solution/
来源：力扣（LeetCode）
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