382. 链表随机节点

import random
from typing import Optional


# 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: Optional['ListNode']):
        self.head = head

    def getRandom(self) -> Optional[int]:
        if self.head is None:
            return None

        target, cur, counter = None, self.head, 0
        while cur is not None:
            if target is None:
                target = cur.val
            elif random.randint(0, counter) < 1:
                target = cur.val

            cur, counter = cur.next, counter + 1
        return target

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