398. Random Pick Index

class Solution {
    private final int[] nums;

    public Solution(int[] nums) {
        this.nums = nums;
    }

    public int pick(int target) {
        int index = -1, bound = 1;
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] == target && ThreadLocalRandom.current().nextInt(bound++) == 0) {
                index = i;
            }
        }

        return index;
    }
}

Reference

398. Random Pick Index