题目：

题解：

class Solution:
    def __init__(self, nums: List[int]):
        self.nums = nums

    def pick(self, target: int) -> int:
        ans = cnt = 0
        for i, num in enumerate(self.nums):
            if num == target:
                cnt += 1  # 第 cnt 次遇到 target
                if randrange(cnt) == 0:
                    ans = i
        return ans