[leetcode]382. Linked List Random Node

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();

思路:参考 Reservoir sampling（水塘抽样）

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) {        this->head = head;    }        /** Returns a random node's value. */    int getRandom() {        int res = head->val;        int i = 2;        auto cur = head->next;        while( cur != nullptr )        {            int j = rand()%i;            if( j == 0 ) res = cur->val;                        i++;            cur = cur->next;        }        return res;    }private:    ListNode* head;};