in LeetCode Reservoir Sampling ~ read.

Linked List Random Node

382. Linked List Random Node

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.

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();  
public class Solution {  
    private int[] nums;
    private Random random;

    public Solution(int[] nums) {
        this.nums = nums;
        this.random = new Random();
    }

    public int pick(int target) {
        int result = -1;
        int count = 0;

        for (int i = 0; i < nums.length; i++) {
            if (nums[i] != target) {
                continue;
            }
            count++;

            if (random.nextInt(count) == 0) {
                result = i;
            }
        }

        return result;
    }
}

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

Hope this helps,
Michael