https://leetcode.com/problems/random-pick-index/public class Solution {    private Map mp;    private Random rand;        public Solution(int[] nums) {        mp = new HashMap();        for (int i=0; i<nums.length; i++) {            List lt = (ArrayList)mp.remove(nums[i]);            if (lt == null) {                lt = new ArrayList();            }            lt.add(i);            mp.put(nums[i], lt);        }        rand = new Random();    }        public int pick(int target) {        List lt = (ArrayList)mp.get(target);        return (int)lt.get(rand.nextInt(lt.size()));    }}/** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(nums); * int param_1 = obj.pick(target); */