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.

分析
最容易想到的方法是，用一個哈希表unordered_map<int, bool> visited，記錄每個元素是否被訪問過，一旦出現某個元素被重複訪問，說明存在環。空間複雜度O(n)，時間複雜度O(N)。
最好的方法是時間複雜度O(n)，空間複雜度O(1)的。設置兩個指針，一個快一個慢，快的指針每次走兩步，慢的指針每次走一步，如果快指針和慢指針相遇，則說明有環。

/**
 * 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) {
        // 設置兩個指針，一個快一個慢
        ListNode slow = head, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) return true;
        }
        return false; 
    }
}