Reservoir sampling is a family of randomized algorithms for randomly choosing k samples from a list of n items, where n is either a very large or unknown number.

Reference

public class Solution {
    public static int[] randamPick(int[] arr, int k) {
        int[] result = new int[k];
        int i = 0;
        for (; i < k; i++) {
            result[i] = arr[i];
        }
        Random rand = new Random();
        for (; i < arr.length; i++) {
            int j = rand.nextInt(i + 1);
            if (j < k) {
                result[j] = arr[i];
            }
        }
        return result;
    }
}

class Solution(object):
    def randam_picket(self, nums, k):
        if not nums or not k:
            return []

        ret = nums[0:k]
        for i, num in enumerate(nums):
            j = random.randint(0, i)
            if j < k:
                ret[i] = num
        return ret