Leetcode Linked List Random Node C++（蓄水池抽样算法）

leetcode上一道典型的关于蓄水池抽样算法，关于蓄水池抽样算法,有一篇博客讲的非常好

http://blog.csdn.net/huagong_adu/article/details/7619665

理清楚了算法思路后

我们来看看这道题的题意

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();

利用蓄水池抽样算法，写出可以AC的代码，这里想多说一句，还是那句话，注意细节，bug往往出现在细节处，还有如果使用MAC的同学想要用gdb调试的话，可以尝试下Xcode中带的命令行工具中的lldb，使用方法与gdb相同。

下面的代码中Solution部分是可以AC的代码，同时还有附带的测试用例。

/***********************
Leetcode 382 Linked List Random Node
algorithms : reserviors sampling
author : xiaolewen_bupt
time complexity : O(N), space complexity : O(1)
test the leetcode's
linkList problem
in the loacal environment
***********************/

#include <stdlib.h>
#include <time.h>
#include <iostream>
using namespace std;
struct ListNode
{
int val;
ListNode *next;
ListNode(int x): val(x),next(NULL) {}
};

void addToTail(ListNode **head,int val)
{
ListNode *node =new ListNode(val);
if (!*head)
{
*head = node;
}
else
{
ListNode *tmp=*head;
while (tmp->next)
tmp=tmp->next;
tmp->next=node;
}

}

void printList(ListNode *head)
{
ListNode* tmp;
tmp=head;
if (tmp==NULL)
cout<<"empty list";
else
{
while (tmp!=NULL)
{
cout<<tmp->val<<',';
tmp=tmp->next;
}
cout<<endl;
}

}

/**
* 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) {
iter = head;
head_temp = head;
}

/** Returns a random node's value. */
int getRandom() {
resetIter(&iter);
//srand(time(NULL));
int res = iter -> val;
iter = iter -> next;
for(int i = 1; iter != NULL; i++)
{
int randomNum = random(0, i);
if(randomNum < 1)
res = iter -> val;
iter = iter -> next;
}
return res;
}
private:
void resetIter(ListNode** iter)  //if you want modify a pointer , please pass the pointer's pointer to the function
{
*iter = head_temp;
}
ListNode *head_temp;
ListNode *iter;
int random(int low, int high)
{
if(low > high)
random(high, low);
int res = low + rand() % (high - low + 1);
return res;
}
};

/**
* Your Solution object will be instantiated and called as such:
* Solution obj = new Solution(head);
* int param_1 = obj.getRandom();
*/
int main()
{
int lt=time(NULL);
srand(lt);
int len = rand()%20;
cout<<len<< endl;
if (len<1)
return 0;
ListNode *root=NULL;
for (int i=0;i<len;i++)
addToTail(&root,rand()%100);
printList(root);
// fill the parameters in your function below
Solution* solution = new Solution(root);
cout << solution -> getRandom() << endl;
cout << solution -> getRandom() << endl;
cout << solution -> getRandom() << endl;
cout << solution -> getRandom() << endl;
delete solution;
}