382. Linked List Random Node

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:

    def __init__(self, head: ListNode):
        """ tc O(N) sc O(N)
        @param head The linked list's head.
        Note that the head is guaranteed to be not null, so it contains at least one node.
        """
        self.A = []
        cur = head
        while cur:
            self.A.append(cur.val)
            cur = cur.next
        self.n = len(self.A)

    def getRandom(self) -> int:
        """ tc O(1)  sc O(N)
        Returns a random node's value.
        """
        idx = random.randint(0,self.n-1)
        return self.A[idx]
        


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

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:

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

    def getRandom(self) -> int:
        """ tc O(N)  sc O(1)
        Returns a random node's value.
        """
        n,k = 1,1
        res = 0
        cur = self.head
        while cur:
            if random.random() < k/n:
                res = cur.val
            cur = cur.next
            n += 1 
        return res