Two Sum Solutions in Java

/// Source : https://leetcode.com/problems/two-sum/description/
/// Author : liuyubobobo
/// Time   : 2017-11-15

import java.util.HashMap;

/// Brute Force
/// Time Complexity: O(n^2)
/// Space Complexity: O(1)
public class Solution1 {

    public int[] twoSum(int[] nums, int target) {

        for(int i = 0 ; i < nums.length; i ++)
            for(int j = 0 ; j < nums.length ; j ++)
                if(nums[i] + nums[j] == target){
                    int[] res = {i, j};
                    return res;
                }

        throw new IllegalStateException("the input has no solution");
    }

    private static void printArr(int[] nums){
        for(int num: nums)
            System.out.print(num + " ");
        System.out.println();
    }

    public static void main(String[] args) {

        int[] nums = {0, 4, 3, 0};
        int target = 0;
        printArr((new Solution1()).twoSum(nums, target));
    }
}

/// Source : https://leetcode.com/problems/two-sum/description/
/// Author : liuyubobobo
/// Time   : 2017-11-15
/// Updated: 2018-12-28

import java.util.HashMap;

/// Two-Pass Hash Table
/// Time Complexity: O(n)
/// Space Complexity: O(n)
public class Solution2 {

    public int[] twoSum(int[] nums, int target) {

        HashMap<Integer, Integer> record = new HashMap<Integer, Integer>();
        for(int i = 0 ; i < nums.length ; i ++)
            record.put(nums[i], i);

        for(int i = 0 ; i < nums.length; i ++){

            Integer index = record.get(target - nums[i]);
            if(index != null && index != i){
                int[] res = {i, index};
                return res;
            }
        }

        throw new IllegalStateException("the input has no solution");
    }

    private static void printArr(int[] nums){
        for(int num: nums)
            System.out.print(num + " ");
        System.out.println();
    }

    public static void main(String[] args) {

        int[] nums = {0, 4, 3, 0};
        int target = 0;
        printArr((new Solution2()).twoSum(nums, target));
    }
}

/// Source : https://leetcode.com/problems/two-sum/description/
/// Author : liuyubobobo
/// Time   : 2017-11-15

import java.util.HashMap;

/// One-Pass Hash Table
/// Time Complexity: O(n)
/// Space Complexity: O(n)
public class Solution3 {

    public int[] twoSum(int[] nums, int target) {

        HashMap<Integer, Integer> record = new HashMap<Integer, Integer>();
        for(int i = 0 ; i < nums.length; i ++){

            int complement = target - nums[i];
            if(record.containsKey(complement)){
                int[] res = {i, record.get(complement)};
                return res;
            }

            record.put(nums[i], i);
        }

        throw new IllegalStateException("the input has no solution");
    }

    private static void printArr(int[] nums){
        for(int num: nums)
            System.out.print(num + " ");
        System.out.println();
    }

    public static void main(String[] args) {

        int[] nums = {0, 4, 3, 0};
        int target = 0;
        printArr((new Solution3()).twoSum(nums, target));
    }
}