Linked List Random Node

Intuition

Solution

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
        self.n = 0
        index = self.head
        while index is not None:
            self.n += 1
            index = index.next

    def getRandom(self) -> int:
        """
        Returns a random node's value.
        """
        n = random.randint(0, self.n - 1)
        index = self.head
        for _ in range(n):
            index = index.next
        return index.val


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