TwoSum.java

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

/**
 * Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.
 * You may assume that each input would have exactly one solution, and you may not use the same element twice.
 * You can return the answer in any order.
 *
 *
 * Example 1:
 *
 * Input: nums = [2,7,11,15], target = 9
 * Output: [0,1]
 * Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].
 * Example 2:
 *
 * Input: nums = [3,2,4], target = 6
 * Output: [1,2]
 * Example 3:
 *
 * Input: nums = [3,3], target = 6
 * Output: [0,1]
 *
 *
 * Constraints:
 *
 * 2 <= nums.length <= 104
 * -109 <= nums[i] <= 109
 * -109 <= target <= 109
 * Only one valid answer exists.
 *
 *
 * Follow-up: Can you come up with an algorithm that is less than O(n2) time complexity?
 */

 public class TwoSum {
    public static void main(String[] args) {
        int[] nums = {2,7,11,15};
        int target = 9;
        final TwoSum sum = new TwoSum();
        int[] ans = sum.twoSum(nums, target);
        System.out.println("" + ans[0] + "," + ans[1]);
    }

     public int[] twoSum(int[] nums, int target) {
        final Map<Integer, Integer> map = new HashMap<>();
        final int[] answer = new int[2];
        for (int index = 0; index < nums.length; index++) {
            if(map.containsKey(target - nums[index])) {
                answer[0] = map.get(target - nums[index]);
                answer[1] = index;
                break;
            } else {
                map.put(nums[index], index);
            }
        }
        return answer;
    }
 }