LeetCode - 141. Linked List Cycle

題目：

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

思路與步驟：

採用“快慢指針”查檢查鏈表是否含有環。讓一個指針一次走一步，另一個一次走兩步，如果鏈表中含有環，快指針會再次和慢指針相遇。

編程實現:

/**
 * 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 || head.next==null) return false;
        ListNode fast = head;
        ListNode slow = head;
        while(fast.next!=null && fast.next.next!=null){
            fast = fast.next.next;
            slow = slow.next;
            if(fast==slow)  return true;
        }
        return false;
    }
}

注意循環條件！否則會有異常

while(fast.next!=null && fast.next.next!=null)