LeetCode 141. Linked List Cycle （链表循环）

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

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题目标签：Linked List

　　题目给了我们一个 Linked List，让我们判断它是否循环。

　　利用快，慢指针，快指针一次走2步，慢指针一次走1步，如果循环，快慢指针一定会相遇。

Java Solution:

Runtime beats 98.15% 

完成日期：06/09/2017

关键词：singly-linked list；cycle

关键点：fast, slow pointers

/**
 * 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;
        ListNode fast = head;
        
        do
        {
            if(fast.next == null || fast.next.next == null) // if fast reaches to end, it doens't have a cycle
                return false;
            
            fast = fast.next.next; // fast moves 2 steps
            slow = slow.next; // slow moves 1 step
            
            
        } while(fast != slow);
        
        return true; // if fast catches slow, meaning it has a cycle
    }
}

参考资料：N/A

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