Linked List Cycle
Given a linked list, determine if it has a cycle in it.
To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
Example 1:
Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.
Example 2:
Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.
Example 3:
Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.
Follow up:
Can you solve it using O(1) (i.e. constant) memory?
題意
判斷連結清單是否有環
思路
快慢指針，慢指針一次走一步，快指針一次走兩步，如果連結清單有環，則快指針和慢指針會相遇。時間複雜度O(n)，空間複雜度O(1).
代碼
/*
* @lc app=leetcode id=141 lang=java
*
* [141] Linked List Cycle
*/
/**
* 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 slow = head, fast = head;
while (fast != null) {
slow = slow.next;
fast = fast.next;
if (fast != null) {
fast = fast.next;
} else {
break;
}
if (slow == fast) {
return true;
}
}
return false;
}
}