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();

Solution：

思路：

Time Complexity: O(N) Space Complexity: O(N)

Solution Code：

public class Solution {
    
    ListNode head;
    Random random;
    
    public Solution(ListNode h) {
        head = h;       
        random = new Random();        
    }
    
    public int getRandom() {
        
        ListNode c = head;
        int r = c.val;
        for(int i=1;c.next != null;i++){
            
            c = c.next;
            if(random.nextInt(i + 1) == i) r = c.val;                        
        }
        
        return r;
    }
}