141 Linked List Cycle

方法一：使用哈希表存储每次经过的节点。

方法二：使用快慢指针的方法判断。

Python

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def __init__(self):
        self.hash = {}
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        while head and head not in self.hash:
            self.hash[head] = 1
            head = head.next
        if head == None:
            return False
        else:
            return True

Java

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;
        }
        slow = slow.next;
        fast = fast.next.next;
    }
    return true;
}