[382]Linked List Random Node

【题目描述】

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();

【解题思路】

https://discuss.leetcode.com/topic/55049/java-solution-with-cases-explain

【代码】

/** * 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. */    ListNode* head;    Solution(ListNode* head) {        this->head=head;    }    int randomnum(int x,int y){        return x+(int)(random()%(y-x+1));    }    /** Returns a random node's value. */    int getRandom() {        ListNode* c=head;        int res=c->val;        for(int i=1;c->next!=NULL;i++){            c=c->next;            if(i==randomnum(0,i)){                res=c->val;                //break;            }        }        return res;    }};/** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(head); * int param_1 = obj.getRandom(); */