/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
 
		int []arr = new int[]{1,1,2,2,3,4,5,6};
		int k = 6;
		System.out.println(countAllPairsWithSumK(arr,k));
	}
	static int countAllPairsWithSumK(int[] arr, int k){
		HashMap<Integer,Integer> map = new HashMap<>();
		int cnt = 0;
		for(int i=0;i<arr.length;i++){
			if(map.containsKey(k-arr[i])){
				cnt+=map.get(k-arr[i]);
			}
			map.put(arr[i],map.getOrDefault(arr[i],0)+1);
		}
		return cnt;
	}
}

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