398. Random Pick Index

Given an array of integers with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array.
Note:
The array size can be very large. Solution that uses too much extra space will not pass the judge.
Example:
int[] nums = new int[] {1,2,3,3,3};
Solution solution = new Solution(nums);

// pick(3) should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
solution.pick(3);

// pick(1) should return 0. Since in the array only nums[0] is equal to 1.
solution.pick(1);




class Solution {
    int[] nums;
    Random rand;

    public Solution(int[] nums) {
        this.nums = nums;
        this.rand = new Random();
    }
    
    public int pick(int target) {
        int count = 0;
        int sample = -1;
        for(int i = 0; i < nums.length; i++){
            if(nums[i] == target){
                count++;
                int x = rand.nextInt(count);
                if(x == 0){
                    sample = i;
                }
            }
        }
        return sample;
    }
}