Java实现判断环形链表

• LeetCode算法第141题，是判断环形链表。
• 其实这题算是比较简单，思路也不复杂，这次就把Java的代码简单实现一下。

private static boolean test1 (ListNode head) {
    int i = 0;
    boolean fal = false;
    ListNode next = head;
    while (next != null) {
        if (i > 10000000) {
            fal = true;
            break;
        }
        i++;
        next = next.next;
    }
    return fal;
}

private static boolean test2 (ListNode head) {
    Set set = new HashSet<>();
    boolean fal = false;
    ListNode next = head;
    while (next != null) {
        if (set.contains(next)) {
            fal = true;
            break;
        }
        set.add(next);
        next = next.next;
    }
    return fal;
}

private static boolean test3 (ListNode head) {
    boolean fal = false;
    // 快、慢指针
    ListNode index = head;
    ListNode next = head;
    while (next != null) {
        ListNode toIndex;
        if (null == index || (toIndex = index.next) == null || (index = toIndex.next) == null) {
            break;
        }
        next = next.next;
        if (next == index) {
            fal = true;
            break;
        }
    }
    return fal;
}

public class Test1 {

    
    public static void main (String[] args) throws IOException {

        final ListNode listNode = buildNode();

        // 方式一：无限循环
        System.out.println("方式一：无限循环：" + test1(listNode));

        // 方式二：hash表
        System.out.println("方式二：hash表：" + test2(listNode));

        // 方式三：双指针
        System.out.println("方式三：双指针：" + test3(listNode));
    }


    
    private static ListNode buildNode () {
        ListNode first = new ListNode(1);
        ListNode next = first;
        ListNode linkListNode = null;
        for (int i = 2; i < 100000; i++) {
            ListNode listNode = new ListNode(i);
            next.setNext(listNode);
            next = listNode;
            if (i == 70000) {
                linkListNode = listNode;
            }
        }
        // 随机制作环
        next.setNext(linkListNode);
        return first;

    }

    
    @Data
    private static class ListNode {
        private ListNode next;
        private Integer value;

        public ListNode (Integer value) {
            this.value = value;
        }

        @Override
        public int hashCode () {
            return value.hashCode();
        }
    }
}

• 虚心学习，共同进步 -_-