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382.linked-list-random-node.py
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97 lines (82 loc) · 2.48 KB
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# Tag: Linked List, Math, Reservoir Sampling, Randomized
# Time: O(N)
# Space: O(1)
# Ref: -
# Note: -
# Given a singly linked list, return a random node's value from the linked list. Each node must have the same probability of being chosen.
# Implement the Solution class:
#
# Solution(ListNode head) Initializes the object with the head of the singly-linked list head.
# int getRandom() Chooses a node randomly from the list and returns its value. All the nodes of the list should be equally likely to be chosen.
#
#
# Example 1:
#
#
# Input
# ["Solution", "getRandom", "getRandom", "getRandom", "getRandom", "getRandom"]
# [[[1, 2, 3]], [], [], [], [], []]
# Output
# [null, 1, 3, 2, 2, 3]
#
# Explanation
# Solution solution = new Solution([1, 2, 3]);
# solution.getRandom(); // return 1
# solution.getRandom(); // return 3
# solution.getRandom(); // return 2
# solution.getRandom(); // return 2
# solution.getRandom(); // return 3
# // getRandom() should return either 1, 2, or 3 randomly. Each element should have equal probability of returning.
#
#
# Constraints:
#
# The number of nodes in the linked list will be in the range [1, 104].
# -104 <= Node.val <= 104
# At most 104 calls will be made to getRandom.
#
#
# Follow up:
#
# What if the linked list is extremely large and its length is unknown to you?
# Could you solve this efficiently without using extra space?
#
#
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
import random
class Solution:
def __init__(self, head: Optional[ListNode]):
self.head = head
cur = head
count = 0
while cur is not None:
cur = cur.next
count += 1
self.count = count
def getRandom(self) -> int:
t = random.randint(1, self.count)
res = self.head
for i in range(1, t):
res = res.next
return res.val
import random
class Solution:
def __init__(self, head: Optional[ListNode]):
self.head = head
def getRandom(self) -> int:
res = self.head
node = res.next
i = 1
while node is not None:
if (random.randint(0, i) == 0):
res = node
i += 1
node = node.next
return res.val
# Your Solution object will be instantiated and called as such:
# obj = Solution(head)
# param_1 = obj.getRandom()