leetcode141~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?

解決鏈表問題的經典做法：使用倆指針fast和slow，判斷最後fast是爲null還是fast==slow即可。

public class LinkedListCycle {
    /*
     * 思路：使用 兩個指針slow和fast 看最後有沒有相遇
     */
    public boolean hasCycle2(ListNode head) {
        if(head==null) return false;
        ListNode slow = head;
        ListNode fast = head;
        boolean flag = false;
        while(fast!=null && fast.next!=null) {
            slow = slow.next;
            fast = fast.next.next;
            if(slow == fast) {
                flag = true;
                break;
            }
        }
        return flag;
    }

    public boolean hasCycle(ListNode head) {
        if(head==null || head.next == null) return false;
        ListNode slow = head;
        ListNode fast = head.next;
        while(slow!=fast) {
            if(fast==null || fast.next==null) {
                return false;
            }
            fast = fast.next.next;
            slow = slow.next;
        }
        return true;
    }
}