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 {
    List<Integer> indlist;
    List<Integer> numlist;
    public Solution(int[] nums) {
        indlist = new ArrayList();
        numlist = new ArrayList();
        for(int i: nums) numlist.add(i);
    }
    
    public int pick(int target) {
        for(int i = 0; i < numlist.size(); i++){
            if(target == numlist.get(i)) indlist.add(i);
        }
        int le = indlist.size();
        int res = 0;
        Random r = new Random();
        return indlist.get(r.nextInt(le));
    }
}

Random r = new Random()

r.nextInt(m) return an integer from [0, m)