Share my simple Java solution

  • 2

    If we have int[] nums = [1,2,3,1,1,1,1,1], using r.nextInt(n) will give us a random int between [0, n) (exclusive), so each number in nums has 1/n probability to be returned.

    Thus, numbers equal to target share the same probability.

    public class Solution {
        int[] nums;
        Random r;
        public Solution(int[] nums) {
            this.nums = nums;
            r = new Random();
        public int pick(int target) {
            int size = nums.length;
            int i = r.nextInt(size);
            while (nums[i] != target) {
                i = r.nextInt(size);
            return i;

  • 0

    Correct me if I'm wrong.
    I think your code not reservoir sampling.
    It says "The array size can be very large", which means the input is a stream with infinite length. So you cannot assume length is known. The reservoir sampling algorithm does not require length of input. Pls check this wiki page:

  • 0

    Hi @BBQ, Thanks for pointing out! I think you are right.

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