LeetCode-in-Java

382. Linked List Random Node

Medium

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 integer array nums.
• 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 choosen.

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?

Solution

import com_github_leetcode.ListNode;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;

/*
 * 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; }
 * }
 */
@SuppressWarnings("java:S2245")
public 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();
 */

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