Linked List Cycle 有环的链表@LeetCode

经典快慢指针找环问题

package Level2;

import Utility.ListNode;

/**
 * Linked List Cycle
 * 
 * Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?
 *
 */
public class S127 {

	public static void main(String[] args) {

	}
	
	// 经典快慢指针找环问题
	public boolean hasCycle(ListNode head) {
        ListNode slow = head, fast = head;
        
        while(true){
        	if(fast==null || fast.next==null || slow==null){
        		return false;
        	}
        	
        	fast = fast.next.next;
        	slow = slow.next;
        	
        	if(fast == slow){
        		return true;
        	}
        }
    }

}

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