Number of Pairs which sum equal to the target

Question:

Find the Number of Pairs which sum is equal to the given target

Example:

[1, 5, 4, 3, 3, 7, 1, 2]

Target = 6

Output: 4    Pairs (1,5) (5,1) (4,2) (3,3)

Number of Pairs in an array:

Approach 1:

Logic:

• Initialise the Hash-Map
• Check the element in Hash-Map which is subtracted the from array element with target value
• If element found then get the value of the key from Hash-Map and add into the count
• Otherwise insert element into the Hash-Map with value of 1
• If already present increment the key’s value
• At last print the count of pairs present in array

Explanation:

Program:

Java:

import java.util.HashMap;

public class FindTargetPairInArray{
  
static void findTargetPairInArray(int array[], int target)
{
    HashMap<Integer,Integer> element = new HashMap<>();

    int length = array.length;
    int count = 0;
    int value = 0;

    for (int i = 0; i < length; i++) {

        if (element.containsKey(target - array[i])) {

	    //add the count with value of HashMap
            count += element.get(target - array[i]);
            value = element.get(target - array[i]);
		
	    //Print the pairs
            while(value != 0)
            {
                 System.out.println("(" + array[i] + ", " +(target - array[i])+")");
                 value--;
                    
            }
            
        }
	//Insert the key into the HashMap
        if(element.containsKey(array[i])){
            element.put(array[i], element.get(array[i])+1);
        }
        else{
            element.put(array[i], 1);
        }
    }

    System.out.println("Count of pairs: "+ count );
    
}
  
public static void main(String[] args)
{
    int array[] = { 1, 5, 4, 3, 3, 7, 1, 2};
    int target = 6;

    findTargetPairInArray(array, target);
}
}

Approach 2:

Logic:

• Check the element is greater than or equal to target
• If yes then no need to do below operations, continue with next element
• Add every element with every other element in the array
• Then check the value equal to target or not
• If yes increment the count and at last print the Number of Pairs

Explanation:

Program:

C:

#include <stdio.h>

void main()
{
	int arr[] = { 1, 5, 4, 3, 3, 7, 1, 2};
	int target = 6;
	int count = 0;

	int length = sizeof(arr)/sizeof(arr[0]);

	//Check the pairs
	for(int i = 0; i < length - 1; i++)
	{

		//skip if element is greater than or equal to target
		if(arr[i] >= target)
			continue;

		for(int j = i + 1; j < length; j++)
		{
			if(arr[i] + arr[j] == target)
			{
				count++;
				printf("(%d %d)", arr[i], arr[j]);
			}
		}
	}

	printf("\nCount of pairs: %d",count);
}

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