Print multiple missing elements in an array

Java/multiple_dup.java

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+/*A number of balls are kept in a basket each named in the range [1,n]. some students picked few balls randomly and changed
+their numbers to existing numbers in the box and then added back. Now box has unique numbers + duplicate numbers.
+Find the original numbers on the balls whose numbers were changed.
+
+Input variables:-
+   int t: no. of test cases
+   int n: range
+   int arr[]: array conatining the original and duplicate numbered balls
+
+Input Description:
+-First line of input consists of t, the number of test cases
+-Next each lines consist of n the range
+-Next line consist of the array conatining the original and duplicate numbered balls
+
+Output Description:-
+   Print the balls whose numbers were changed
+
+Constraints:-
+ -> 1<=t,n<=10
+ ->1<= value of each array element <=10
+
+ Sample Input:
+2
+8
+4 3 2 7 8 2 3 1
+2
+1 1
+
+Sample Output:
+5 6
+2
+ */
+
+
+
+import java.util.*;
+
+public class multiple_dup {
+    public static void main(String[] args) {
+        Scanner s = new Scanner(System.in);
+        int t, i, j, n;
+        t = s.nextInt();
+        for (i = 0; i < t; i++) {
+            n = s.nextInt();
+            int[] arr = new int[n];
+
+            for (j = 0; j < n; j++) {
+                arr[j] = s.nextInt();
+            }
+
+            int[]reg = new int[arr.length+1];
+
+            for (int k:arr){
+                reg[k]=1;
+            }
+
+            for (j=1;j<reg.length;j++)
+            {
+                if (reg[j]==0)
+                {
+                    System.out.print(j+" ");
+                }
+            }
+            System.out.println();
+        }
+    }
+}
+