LeetCode: Linked List Cycle 解题报告

Linked List Cycle

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

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

SOLUTION 1:

经典快慢指针问题。如果存在环，fast, slow必然会相遇。就像2个速度不一样的人在环形跑道赛跑，总有一个时间他们会相遇。

如果fast到达了null，就是没有环，可以返回false.

package Algorithms.list;

import Algorithms.algorithm.others.ListNode;

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