1. two sum

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