LeetCode | 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?

tags提示：two pointers

采用“快慢双指针”的方法来判断：
每次slow_pointer前进一格，而fast_pointer前进两格，如若list存在cycle的话，则快指针一定会与慢指针重合。（因为当list存在cycle时，向后遍历是一个无限循环的过程，在此循环中两指针一定会重合）

public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head==null || head.next==null) return false;
        
        ListNode slow_pointer = head;
        ListNode fast_pointer = head;
        
        while(fast_pointer!=null && fast_pointer.next!=null){   //向后遍历中出现null，则一定无cycle
            slow_pointer = slow_pointer.next;
            fast_pointer = fast_pointer.next.next;
            
            if(slow_pointer == fast_pointer){    //如果快指针与慢指针重合了，则一定有cycle
                return true;
            }
        }
        
        return false;
    }
}