链表（一）

快慢指针法

• 基本算法：https://www.cnblogs.com/songdechiu/p/6686520.html
• 扩展应用：https://www.cnblogs.com/hxsyl/p/4395794.html

判断一个链表是否存在环

• code

/**
 * 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;
        if(head.next.next==null) return false;
        
        ListNode slow=head;
        ListNode fast=head;
        while(fast!=null && fast.next!=null){
             slow=slow.next;
             fast=fast.next.next;
             if(fast==slow){
                 return true;
             }
         }
        return false;
    }
}