描述

給定一個鏈表，判斷它是否有環。

樣例

給出 -21->10->4->5, tail connects to node index 1，返回 true

相關題目

帶環鏈表2 & 兩個鏈表的交叉

代碼實現

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param head: The first node of linked list.
     * @return: True if it has a cycle, or false
     */
     //快 慢 指 針
     //快指針以慢指針二倍的速度移動，如果快慢指針指向同一地址，則返回true
    public boolean hasCycle(ListNode head) {  
        if (head == null || head.next == null) {
            return false;
        }
        ListNode fast = head.next;
        ListNode slow = head;
        while (fast != slow) {
            if (fast == null || fast.next == null) {
                return false;
            } else {
                fast = fast.next.next;
                slow = slow.next;
            }
        }
        return true;
    }
}