leetcode 382. Linked List Random Node 等概率随机获取结点 + 蓄水池算法

Myth丶恋晨 2022-06-07 02:51 137阅读 0赞

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();

蓄水池算法，要是我肯定是想不出来的，所以不会做，网上看到的代码。

代码如下：

/*
    class ListNode 
     {
          int val;
          ListNode next;
          ListNode(int x) { val = x; }
     }
     */
    
    /*
     * 这道题用到的就是这个蓄水池算法。看代码的时候就简单多了。
     */
    class Solution 
    {
        ListNode head;
        public Solution(ListNode head) 
        {
            this.head = head;
        }
    
        public int getRandom() 
        {
            Random random = new Random();
            int result = head.val;
            ListNode currNode = head;
            for(int i=1; currNode.next!=null; i++) 
            {
                currNode = currNode.next;
                if((random.nextInt(i + 1)) == i)
                {
                    result = currNode.val;
                }
            }
            return result;
        }
    }
    
    /**
     * Your Solution object will be instantiated and called as such:
     * Solution obj = new Solution(head);
     * int param_1 = obj.getRandom();
     */

下面是C++做法，

这道题给了我们一个链表，让我们随机返回一个节点，那么最直接的方法就是先统计出链表的长度，然后根据长度随机生成一个位置，然后从开头遍历到这个位置即可，

可以和这道题[leetcode 384. Shuffle an Array 数组随机洗牌][leetcode 384. Shuffle an Array]一起学习

代码如下：

#include <iostream>
    #include <vector>
    #include <map>
    #include <unordered_map>
    #include <set>
    #include <unordered_set>
    #include <queue>
    #include <stack>
    #include <string>
    #include <climits>
    #include <algorithm>
    #include <sstream>
    #include <functional>
    #include <bitset>
    #include <numeric>
    #include <cmath>
    #include <regex>
    
    using namespace std;
    
    
    /*
    struct ListNode 
    {
         int val;
         ListNode *next;
         ListNode(int x) : val(x), next(NULL) {}
    };
    */
    
    
    class Solution 
    {
    public:
    
        int len;
        ListNode *head;
    
    
        /** @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) 
        {
            len = 0;
            ListNode *cur = head;
            this->head = head;
            while (cur) 
            {
                ++len;
                cur = cur->next;
            }
        }
    
        /** Returns a random node's value. */
        int getRandom() 
        {
            int t = rand() % len;
            ListNode *cur = head;
            while (t) 
            {
                --t;
                cur = cur->next;
            }
            return cur->val;
        }
    };

[leetcode 384. Shuffle an Array]: http://blog.csdn.net/jackzhang_123/article/details/78174425