Two Sum with a HashMap

import java.util.*;

public class TwoSum {
    public static void main(String[] args) {
        System.out.println(Arrays.toString(twoSum(new int[]{2, 7, 11, 15}, 9)));    // [0, 1]
        System.out.println(Arrays.toString(twoSum(new int[]{3, 2, 4}, 6)));          // [1, 2]
        System.out.println(Arrays.toString(twoSum(new int[]{3, 3}, 6)));             // [0, 1]
        System.out.println(Arrays.toString(twoSum(new int[]{1, 2}, 10)));            // []
        System.out.println(Arrays.toString(twoSumBruteForce(new int[]{2, 7, 11, 15}, 9)));   // [0, 1]
    }

    // One pass, O(n) time and O(n) space: remember each value's index and look for the complement.
    static int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> indexOf = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            Integer j = indexOf.get(target - nums[i]);
            if (j != null) return new int[]{j, i};
            indexOf.put(nums[i], i);
        }
        return new int[0];
    }

    // Two nested loops, O(n^2) time and O(1) space: the answer to give first, then improve.
    static int[] twoSumBruteForce(int[] nums, int target) {
        for (int i = 0; i < nums.length; i++) {
            for (int j = i + 1; j < nums.length; j++) {
                if (nums[i] + nums[j] == target) return new int[]{i, j};
            }
        }
        return new int[0];
    }
}