[LC] 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 {

    private int[] arr;
    private Random rand;
    
    public Solution(int[] nums) {
        this.arr = nums;
        this.rand = new Random();
    }
    
    public int pick(int target) {
        int count = 0;
        int res = -1;
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] != target) {
                continue;
            }
            count += 1;
            int randNum = rand.nextInt(count);
            if (randNum == 0) {
                res = i;
            }
        }
        return res;
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(nums);
 * int param_1 = obj.pick(target);
 */