linked-list-cycle.py

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

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.



Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.


Example 2:

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.


Example 3:

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.




Follow up:

Can you solve it using O(1) (i.e. constant) memory?

'''


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

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        # 判断单链表中有没有环，等价于判断它有没有相同节点。

        # Approach one 快慢指针检测相同点  O(n) , O(1)
        # fast = slow = head
        # while fast and fast.next:
        #     fast = fast.next.next
        #     slow = slow.next
        #     if fast and slow.val == fast.val:
        #         return True
        # return False

        # Approach two 用字典
        dic = {}
        while head:
            if head in dic:
                return True
            else:
                dic[head] = 1
            head = head.next
        return False