LeetCode[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();

<pre name="code" class="cpp">/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:/** @param head The linked list's head.Note that the head is guaranteed to be not null, so it contains at least one node. */Solution(ListNode* head) {p_Head = head;ListNode* p = p_Head;int count = 0;while (p != NULL){count++;p = p->next;}length = count;}/** Returns a random node's value. */int getRandom() {int random = rand() % length;ListNode* p = p_Head;for (int i = 0; i < random; i++)p = p->next;return p->val;}private:ListNode* p_Head;int length;};/** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(head); * int param_1 = obj.getRandom(); */

Follow up: 水塘抽样

</pre><p><pre name="code" class="cpp">class Solution {public:/** @param head The linked list's head. Note that the head is guanranteed to be not null, so it contains at least one node. */Solution(ListNode* head) {p_Head = head;}/** Returns a random node's value. */int getRandom() {int res = p_Head->val, i = 2;ListNode *p = p_Head->next;while (p != NULL) {int j = rand() % i;if (j == 0) res = p->val;++i, p = p->next;}return res;}private:ListNode*p_Head;};