[Java 101] 1. Two Sum with HashMap as a counter

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i=0; i<nums.length; i++){
            int num = nums[i];
            if (map.containsKey(target-num)) {
                return new int[]{i, map.get(target-num)};
            }
            map.put(num, i);
        }
        return null;
    }
}