Create Pair Sum to 0

Author: akshayavb99

Course 2 - Data Structures in JAVA/Lecture 16 - HashMaps/Pair Sum to 0

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+/*
+Given a random integer array A of size N. Find and print the count of pair of elements in the array which sum up to 0.
+Note: Array A can contain duplicate elements as well.
+
+Input format:
+The first line of input contains an integer, that denotes the value of the size of the array. Let us denote it with the symbol N.
+The following line contains N space separated integers, that denote the value of the elements of the array.
+
+Output format :
+The first and only line of output contains the count of pair of elements in the array which sum up to 0. 
+
+Constraints :
+0 <= N <= 10^4
+Time Limit: 1 sec
+
+Sample Input 1:
+5
+2 1 -2 2 3
+Sample Output 1:
+2
+*/
+import java.util.*;
+
+public class Solution {
+	public static int PairSum(int[] input, int size) {
+		/* Your class should be named Solution
+		 * Don't write main().
+		 * Don't read input, it is passed as function argument.
+		 * Return output and don't print it.
+	 	 * Taking input and printing output is handled automatically.
+        */
+        if (size==0)
+            return 0;
+        
+        HashMap<Integer,Integer> map = new HashMap<>();
+        for(int key:input)
+        {	
+            if(map.containsKey(key))
+        	{
+            	map.put(key,map.get(key)+1);
+        	}
+            else
+            {
+                map.put(key,1);
+            }   
+        }
+        /*
+        for (Integer i: map.keySet())
+        {
+            System.out.println(i + ": " + map.get(i));
+        }
+        System.out.println();
+        */
+        int countPairs=0;
+        for (Integer i: map.keySet())
+        {
+            if (map.containsKey(-i) && i!=0)
+            {
+                //System.out.println("Found the negative of " + i);
+            	countPairs=countPairs+(map.get(i)*map.get(-i));   
+                //System.out.println("Current count of pairs: "+countPairs);
+                //map.remove(i);
+            }
+            //System.out.println();
+        }
+        countPairs=countPairs/2;
+        if (map.containsKey(0))
+        {
+            int val=map.get(0);
+            countPairs=countPairs+(val*(val-1))/2;
+        }
+        return countPairs;
+	}
+}