Leetcode #398. Random Pick Index

[yokostan]

Math.random() solution, but runtime only beats 5%:

class Solution {
    HashMap<Integer, ArrayList<Integer>> map = new HashMap<>();

    public Solution(int[] nums) {
        for (int i = 0; i < nums.length; i++) {
            ArrayList<Integer> list = new ArrayList<>();
            if (map.containsKey(nums[i])) {
                list = map.get(nums[i]);
            }
            list.add(i);
            map.put(nums[i], list);
        }
    }

    public int pick(int target) {
        if (!map.containsKey(target)) return 0;
        ArrayList<Integer> list = map.get(target);
        int len = list.size();
        int rand = (int)(Math.random() * len);
        return list.get(rand);
    }
}

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

With reservoir sampling, runtime beats 90%:

class Solution {
    int[] nums;
    Random rand;

    public Solution(int[] nums) {
        this.nums = nums;
        this.rand = new Random();
    }

    public int pick(int target) {
        int result = -1;
        int count = 0;
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] != target) {
                continue;
            }
            if (rand.nextInt(++count) == 0) {
                result = i;
            }
        }
        return result;
    }
}

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