题目描述:
给定一个单链表,随机选择链表的一个节点,并返回相应的节点值。保证每个节点被选的概率一样。
进阶:
如果链表十分大且长度未知,如何解决这个问题?你能否使用常数级空间复杂度实现?
示例:
// 初始化一个单链表 [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()方法应随机返回1,2,3中的一个,保证每个元素被返回的概率相等。
solution.getRandom();
解法:
/**
* 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;
}
/** Returns a random node's value. */
int getRandom() {
ListNode* cur = head->next;
int res = head->val;
int n = 2;
while(cur != NULL){
int idx = rand()%n;
if(idx == 0){
res = cur->val;
}
cur = cur->next;
n++;
}
return res;
}
};
/**
* Your Solution object will be instantiated and called as such:
* Solution* obj = new Solution(head);
* int param_1 = obj->getRandom();
*/