#Reservoir Sampling : large and unknown size 
# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None
from random import randint
class Solution(object):

    def __init__(self, head):
        """
        @param head The linked list's head.
        Note that the head is guaranteed to be not null, so it contains at least one node.
        :type head: ListNode
        """
        self.__head=head
        

    def getRandom(self):
        """
        Returns a random node's value.
        :rtype: int
        """
        reservoir=self.__head.val
        curr,n=self.__head.next,1
        while curr:
            if randint(1,n+1)==1:
                reservoir=curr.val 
            curr,n=curr.next,n+1
        return reservoir
            
        


# Your Solution object will be instantiated and called as such:
# obj = Solution(head)
# param_1 = obj.getRandom()