题目：

题解：

class Solution {vector<int> &nums;
public:Solution(vector<int> &nums) : nums(nums) {}int pick(int target) {int ans;for (int i = 0, cnt = 0; i < nums.size(); ++i) {if (nums[i] == target) {++cnt; // 第 cnt 次遇到 targetif (rand() % cnt == 0) {ans = i;}}}return ans;}
};