Two Sum Problem Statement

Given an array of integers, return the indices of the two numbers whose sum is equal to a given target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:

Given nums = [2, 7, 11, 15], target = 9.

The output should be [0, 1]. 
Because nums[0] + nums[1] = 2 + 7 = 9.

Two Sum Problem solution in Java

METHOD 1. Naive approach: Use two for loops

Time complexity: O(n^2)

import java.util.HashMap;
import java.util.Scanner;
import java.util.Map;

class TwoSum {

    // Time complexity: O(n^2)
    private static int[] findTwoSum_BruteForce(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[] {};
    }


    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);

        int n = keyboard.nextInt();
        int[] nums = new int[n];

        for(int i = 0; i < n; i++) {
            nums[i] = keyboard.nextInt();
        }
        int target = keyboard.nextInt();

        keyboard.close();

        int[] indices = findTwoSum_BruteForce(nums, target);

        if (indices.length == 2) {
            System.out.println(indices[0] + " " + indices[1]);
        } else {
            System.out.println("No solution found!");
        }
    }
}
# Output
$ javac TwoSum.java
$ java TwoSum
4 2 7 11 15
9
0 1

METHOD 2. Use a HashMap

Time complexity: O(n)

import java.util.HashMap;
import java.util.Scanner;
import java.util.Map;

class TwoSum {
    
    // Time complexity: O(n)
    private static int[] findTwoSum(int[] nums, int target) {
        Map<Integer, Integer> numMap = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int complement = target - nums[i];
            if (numMap.containsKey(complement)) {
                return new int[] { numMap.get(complement), i };
            } else {
                numMap.put(nums[i], i);
            }
        }
        return new int[] {};
    }

    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);

        int n = keyboard.nextInt();
        int[] nums = new int[n];

        for(int i = 0; i < n; i++) {
            nums[i] = keyboard.nextInt();
        }
        int target = keyboard.nextInt();

        keyboard.close();

        int[] indices = findTwoSum(nums, target);

        if (indices.length == 2) {
            System.out.println(indices[0] + " " + indices[1]);
        } else {
            System.out.println("No solution found!");
        }
    }
}
# Output
$ javac TwoSum.java
$ java TwoSum
4 2 7 11 15
9
0 1