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();
1 /** 2 * Definition for singly-linked list. 3 * public class ListNode { 4 * int val; 5 * ListNode next; 6 * ListNode(int x) { val = x; } 7 * } 8 */ 9 public class Solution { 10 List<Integer> list = new ArrayList<>(); 11 12 /** @param head The linked list‘s head. 13 Note that the head is guaranteed to be not null, so it contains at least one node. */ 14 public Solution(ListNode head) { 15 while (head != null) { 16 list.add(head.val); 17 head = head.next; 18 } 19 } 20 21 /** Returns a random node‘s value. */ 22 public int getRandom() { 23 Random rand = new Random(); 24 int n = rand.nextInt(list.size()); 25 return list.get(n); 26 } 27 } 28 29 /** 30 * Your Solution object will be instantiated and called as such: 31 * Solution obj = new Solution(head); 32 * int param_1 = obj.getRandom(); 33 */
原文：http://www.cnblogs.com/amazingzoe/p/6416727.html