leetcode 141. Linked List Cycle

判断链表是否有回环

盗个图

使用新节点
/**
 * 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 blank = new ListNode(-1);
        ListNode temp = head; 
        while (head.next != null) {
            temp = head.next;
            head.next = blank;
            head = temp;
        }
        if (temp == blank) return true;
        return false;    
    }
}

快慢指针，如果有回环，快指针总能追上慢指针
/**
 * 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 || head.next == null) return false;
        ListNode slow = head;
        ListNode fast = head.next;
        while (fast !=null && fast.next != null) {
            if (slow == fast) return true;
            slow = slow.next;
            fast = fast.next..next;
        }
        return false;    
    }
}