＃141 Linked List Cycle

Description

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

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail’s next pointer is connected to. Note that pos is not passed as a parameter.

Return true if there is a cycle in the linked list. Otherwise, return false.

Example

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).

Example 2:

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.

Example 3:

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

Constraints:

The number of the nodes in the list is in the range [0, $10^4$].
$-10^5$ <= Node.val <= $10^5$
pos is -1 or a valid index in the linked-list.

Follow up: Can you solve it using O(1) (i.e. constant) memory?

思路

设置两个指针，一个指针每次往后走1格，另一个指针每次往后走2格，如果存在环的话，这两个指针总有一天会相同

代码

/** * 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;
        if (head.next == null)
            return false;
        
        ListNode pointer1 = head.next;
        ListNode pointer2 = pointer1.next;

        while(pointer2 != null && pointer2.next != null) {
    
            if (pointer1 == pointer2)
                return true;
            pointer1 = pointer1.next;
            pointer2 = pointer2.next.next;
        }
        
        return false;
    }
}