题目：

题解：

class Solution {ListNode *head;public:Solution(ListNode *head) {this->head = head;}int getRandom() {int i = 1, ans = 0;for (auto node = head; node; node = node->next) {if (rand() % i == 0) { // 1/i 的概率选中（替换为答案）ans = node->val;}++i;}return ans;}
};