[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();
 */