lettcode 141/142 环形链表

package com.example.lettcode.dailyexercises;

/**
 * @Class DetectCycle
 * @Description TODO
 * @Author
 * @Date 2020/10/10
 **/
public class DetectCycle {
    static class ListNode {
        int val;
        ListNode next;

        ListNode(int x) {
            val = x;
            next = null;
        }
    }

    /**
     * 141 环形链表
     * */
    public static boolean hasCycle(ListNode head) {
        // 使用快慢指针的方式,找到快慢指针的交点
        ListNode fast = head;
        ListNode slow = head;
        // 因为fast指针已经先判断了，就不用判断slow指针了
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) {
                return true;
            }
        }
        return false;
    }

    /**
     * 142 环形链表II
     * */
    public static ListNode detectCycle(ListNode head) {
        //使用快慢指针的方式,找到快慢指针的交点：
        ListNode fast = head;
        ListNode slow = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) {
                //是带环链表
                break;
            }
        }
        if (fast == null || fast.next == null) {
            //不是带环链表
            return null;
        }
        //带环的情况，设定两个引用，分别从链表头部和fast、slow交点出发，按照相同速度同时往后走
        ListNode cur1 = head;
        ListNode cur2 = fast;
        while (cur1 != cur2) {
            cur1 = cur1.next;
            cur2 = cur2.next;
        }
        //环的入口：
        return cur1;
    }

    public static void main(String[] args) {
        // [3,2,0,-4] 1
        ListNode head = new ListNode(3);
        ListNode node2 = new ListNode(2);
        head.next = node2;
        ListNode node3 = new ListNode(0);
        node2.next = node3;
        ListNode node4 = new ListNode(-4);
        node3.next = node4;
        node4.next = node2;
        ListNode listNode = detectCycle(head);
        System.out.println("DetectCycle demo01 result:" + listNode.val);
    }
}