[Java]LeetCode141. 环形链表 | Linked List Cycle

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Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

---

给定一个链表，判断链表中是否有环。

进阶：
你能否不使用额外空间解决此题？

---

 0ms 

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head == null) return false;
        ListNode fast = head;
        ListNode slow = head;
        do{
            if(fast.next == null || fast.next.next == null) return false;
            fast = fast.next.next;
            slow = slow.next;
        }while(fast != slow);
        return true;
    }
}

---

0ms

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
    if(head==null) return false;
    ListNode walker = head;
    ListNode runner = head;
    while(runner.next!=null && runner.next.next!=null) {
        walker = walker.next;
        runner = runner.next.next;
        if(walker==runner) return true;
    }
    return false;
}
}

---

1ms

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
public static boolean hasCycle(ListNode head) {
        if (head == null || head.next == null) {
            return false;
        }
        ListNode slow = head;
        ListNode fast = head.next;
        int f = 1;//上面两行赋值已前进了一次
        while (slow != fast) {
            if (fast == null || fast.next == null) {
                return false;
            }
            slow = slow.next;
            fast = fast.next.next;
            f++;
        }
        System.out.println("LinkedList.size:"+f);
        return true;
    }
}

---

4ms

public class Solution {
    public boolean hasCycle(ListNode head) {
        HashSet<ListNode> visited = new HashSet<>();
        
        ListNode curr = head;
        while (curr != null) {
            if (visited.contains(curr))
                return true;
            visited.add(curr);
            curr = curr.next;
        }
        
        return false;
    }
}

---

5ms

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        Set<ListNode> set = new HashSet<>();
        while(head != null){
            if(set.contains(head)){
                return true;
            } else{
                set.add(head);
            }
            head = head.next;
        }
        return false;
    }
}