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?

[解题思路]

双指针问题，fast指针每次走两步，slow指针每次走一步，如果fast指针可以"追上"slow指针，则说明存在环

public class Solution {
    public boolean hasCycle(ListNode head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(head == null){
            return false;
        }
        
        ListNode fast = head, slow = head;
        while(fast.next != null && fast.next.next != null){
            fast = fast.next.next;
            slow = slow.next;
            if(fast == slow){
                break;
            }
        }
        return (fast.next == null || fast.next.next == null) ? false : true;
    }
}