LinkedListRandomNode382.java

/**
 * 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();
 */

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

public class LinkedListRandomNode382 {
    class Solution {
        private ListNode head;
        private Random rand = new Random();
        /** @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;
        }

        /** Returns a random node's value. */
        public int getRandom() {
            int count = 0;
            int res = -1;
            ListNode p = head;
            while (p != null) {
                count++;
                if (count == 1) {
                    res = p.val;
                } else {
                  int idx = rand.nextInt(count);
                    if (idx == count - 1) res = p.val;
                }
                p = p.next;
            }
            return res;
        }
    }

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

}