382 Linked List Random Node

382. Linked List Random Node

1. Question

Given a singly linked list, return a random node's value from the linked list. Each node must have thesame probabilityof being chosen.

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?

Example:

// Init a singly linked list [1,2,3].
ListNode head = new ListNode(1);
head.next = new ListNode(2);
head.next.next = new ListNode(3);
Solution solution = new Solution(head);

// getRandom() should return either 1, 2, or 3 randomly. Each element should have equal probability of returning.
solution.getRandom();

2. Implementation

(1) Reservoir Sampling

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    Random rand;
    ListNode head;

    /** @param head The linked list's head.
        Note that the head is guaranteed to be not null, so it contains at least one node. */
    public Solution(ListNode head) {
        this.head = head;
        rand = new Random();
    }

    /** Returns a random node's value. */
    public int getRandom() {
        ListNode curNode = head;
        int res = head.val;

        for (int i = 1; curNode != null; i++, curNode = curNode.next) {
            if (rand.nextInt(i) == 0) {
                res = curNode.val;
            }
        }
        return res;
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(head);
 * int param_1 = obj.getRandom();
 */

3. Time & Space Complexity

Reservoir Sampling: 时间复杂度O(n), 空间复杂度O(1)