Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
使用快慢两个指针，慢指针每次前进一步，快指针每次前进两步，如果慢指针和快指针相等，说明有环。
/**
* 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) {
ListNode slow = head;
ListNode fast = head;
while (fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
if (slow == fast) {
return true;
}
}
return false;
}
}