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.

Trick

• Initialize empty hash map. Hash map will contain the value in the array and its index
• Iterate over the array and calculate the difference between target and current value. If map contains key (which is the desired difference), it’s a match! Return the array with both indexes.

Code:

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

        Map<Integer, Integer> map = new HashMap<>();

        for (int i = 0; i < nums.length; i++) {
            int difference = target - nums[i];
            if (map.containsKey(difference)) {
                int[] result = new int[2];
                result[0] = map.get(difference);
                result[1] = i;
                return result;
            }
            map.put(nums[i], i);
        }

        return new int[2];

    }
}