Difficulty

Medium

Question

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?

Link

Linked List Random Node

Solution

Solution implements one-time mapping of the whole linked list into array of pointers of known length. After that, getting random numbers is O(1) time. Saving pointers, not copying values of nodes, ensures O(1) space complexity.

Time complexity of creating solution: O(n) Time complexity of getting random element: O(1) Space complexity: O(1)