random-pick-index

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);
 */