[leetcode]382. Linked List Random Node

[leetcode]382. Linked List Random Node
Analysis
国慶節を待つ!!![ふふ~]
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? チェーンテーブルが長い可能性があることを考慮して、池のサンプリング方法で解決すべきで、ランダムに1つの数を選ぶだけで、容量が1の池に相当するからです.池のサンプリングについては、以下を参照してください.https://blog.csdn.net/My_Jobs/article/details/48372399
Implement

/**
 * 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) {
        ini_head = head;
    }
    
    /** Returns a random node's value. */
    int getRandom() {
        int res = ini_head->val;
        ListNode* tmp = ini_head;
        int i=1;
        while(tmp){
            if(rand() % i == 0)
                res = tmp->val;
            tmp = tmp->next;
            i++;
        }
        return res;
    }
private:
    ListNode* ini_head;
};

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