How to write an algorithm to check if the sum of any two numbers in an array/list matches a given number?

Question

How can I write an algorithm to check if the sum of any two numbers in an array/list matches a given number with a complexity of nlogn?

Answer 1

This question is missing some more details into it. Like what is the return value, the limitation on the input. I have seen some questions related to that, which can be this question with extra requirement, to return the actual elements that result in the input

Here is my version of the solution, it should be O(n).

import java.util.*;

public class IntegerSum {

    private static final int[] NUMBERS = {1,2,3,4,5,6,7,8,9,10};

    public static void main(String[] args) {
        int[] result = IntegerSum.isSumExist(7);
        System.out.println(Arrays.toString(result));
    }

    /**
     * n = x + y
     * 7 = 1 + 6
     * 7 - 1 =  6
     * 7 - 6 = 1
     * The subtraction of one element in the array should result into the value of the other element if it exist;
     */
    public static int[] isSumExist(int n) {
        // validate the input, based on the question
        // This to return the values that actually result in the sum. which is even more tricky
        int[] output = new int[2];
        Map resultsMap = new HashMap();
        // O(n)
        for (int number : NUMBERS) {
            if ( number > n )
                throw new IllegalStateException("The number is not in the array.");
            if ( resultsMap.containsKey(number) ) {
                output[0] = number;
                output[1] = (Integer) resultsMap.get(number);
                return output;
            }
            resultsMap.put(n - number, number);
        }
        throw new IllegalStateException("The number is not in the array.");
    }
}