Question

Please follow the instructions carefully and complete the code given below.

Language: Java

Instructions from your teacher:

(This is a version of an interview question I once saw.) In this problem, we will write a program that, given an integer k, an integer n, and a list of integers, outputs the number of pairs in in the list that add to k.

To receive full credit for design, your algorithm must have a runtime in O(n) , where n is the length of the list of integers. Hint: you can assume that the contains in a HashSet is O(1). Your program should take as input a number k on its own line, followed by a number n on its own line, followed by a space-separated list of n integers. It should then output the number of pairs in the list that add to k. Order does not matter when considering a pair (in other words, in the list [2,1] there is one distinct pair that sums to 3, not two). You may assume that all numbers in the input are distinct. For example, if our file input.txt is:

1

6

1 2 3 4 -2 -3

then we should print 2 since there are two pairs that sum to 1: 3 + (-2) and 4 + (-3).

As another example, if input.txt is:

3

4

1 2 3 4

then we should print 1since there is one pair that sums to 1: 1+2.

The problem PairFinder provides starter code for reading and printing; your task is to fill in thefindPairs()method.

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import java.util.Scanner;

public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
  
// Read in the value of k
int k = Integer.parseInt(sc.nextLine());
  
// Read in the value of n
int n = Integer.parseInt(sc.nextLine());
  
// Read in the list of numbers
int[] numbers = new int[n];
String input = sc.nextLine();
if (input.equals("")) {
numbers = new int[0];
} else {
String[] numberStrings = input.split(" ");
for (int i = 0; i < n; i++) {
numbers[i] = Integer.parseInt(numberStrings[i]);
}
}
  
System.out.println(findPairs(numbers, k));
}
  
private static int findPairs(int[] numbers, int k) {
// TODO fill in this function
throw new UnsupportedOperationException();
}
  
}

Answer 1

Thanks for the question.

Below is the code you will be needing  Let me know if you have any doubts or if you need anything to change.

Thank You !!

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import java.util.Arrays;
import java.util.Scanner;

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

// Read in the value of k
        int k = Integer.parseInt(sc.nextLine());

// Read in the value of n
        int n = Integer.parseInt(sc.nextLine());

// Read in the list of numbers
        int[] numbers = new int[n];
        String input = sc.nextLine();
        if (input.equals("")) {
            numbers = new int[0];
        } else {
            String[] numberStrings = input.split(" ");
            for (int i = 0; i < n; i++) {
                numbers[i] = Integer.parseInt(numberStrings[i]);
            }
        }

        System.out.println("Pairs found: "+findPairs(numbers, k));
    }

    private static int findPairs(int[] numbers, int k) {

        Arrays.sort(numbers);
        int start = 0;
        int end = numbers.length - 1;
        int count = 0;
        while (start <= end) {

            int total = numbers[start] + numbers[end];
            if (total == k) {
                count++;
                start++;
                end--;
            } else if (total > k) end--;
            else start++;

        }
        return count;

    }

}

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