【LeetCode】Linked List Cycle

称号

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

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

解答

首先要注意空链表不成环;不能开额外的空间，即空间复杂度是o(1),可採用“快慢指针”查检查链表是否含有环，假设在快的指针可以追上慢的指针，则有环。否则无环。

/**
 * 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;
        while(fast!=null&&fast.next!=null){  //仅仅要推断快指针是否为空就能够了，可是注意当fast是为结点时，fast.next为空。若推断fast.next.next则会报NullPointerException
            slow=slow.next;
            fast=fast.next.next;
            if(slow==fast){
                return true;
            }
        }
        return false;
    }
}

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