public int[] twoSum2(int[] numbers, int target) { // O(n) solution
        Map<Integer, Integer> numMap = new HashMap<Integer, Integer>();
		int [] res = new int[2];
		for(int i = 0; i < numbers.length; ++i){
			if(numMap.containsKey(target - numbers[i])){
				res[0] = numMap.get(target - numbers[i]) + 1;
				res[1] = i + 1;
				return res;
			}else{
				numMap.put(numbers[i], i);
			}
		}
		return res;
	}