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

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* phead;
    int n;
    /** @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) {
        phead = new ListNode(head->val);
        ListNode *p = head, *pp = phead;
        n = 1;
        while ( p->next != NULL ) {
            int x = p->next->val;
            pp->next = new ListNode(x);
            p = p->next;
            pp = pp->next;
            n += 1;
        }
    }

    /** Returns a random node's value. */
    int getRandom() {
        int target = random()%n + 1;
        ListNode* p = phead;
        for ( int i = 1; i < target ; i++ ) {
            p = p->next;
        }
        return p->val;
    }
};

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

Notes
reservoir sampling?
http://www.cnblogs.com/grandyang/p/5756148.html