382. Linked List Random Node

Problem

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, 10^4].

• -10^4 <= Node.val <= 10^4

• At most 10^4 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?

Solution

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    private List<Integer> al;
    private Random rand;

    public Solution(ListNode head) {
        al = new ArrayList<>();
        rand = new Random();
        while (head != null) {
            al.add(head.val);
            head = head.next;
        }
    }

    public int getRandom() {
        /*
        Math.random() will generate a random number b/w 0 & 1.
        then multiply it with the array size.
        take only the integer part which is a random index.
        return the element at that random index.
         */
        int ind = rand.nextInt(al.size());
        return al.get(ind);
    }
}

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

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).