Question

What is the best approach to solve the following problem in Java?
There are 3 two-dimensional arrays that have equal number of rows and columns:

Array1:
[1]: {1, 2, 3}
[2]: {2, 2, 4}
[3]: {1, 1, 1}

Array2:
[1]: {1, 2, 0}
[2]: {1, 2, 3}
[3]: {1, 0, 1}

Array3:
[1]: {1, 1, 1}
[2]: {1, 2, 3}
[3]: {1, 0, 1}

I need to find out the indexes of rows that exist in all arrays. In the above example the answer should be: [1],[2],[2]

Array1
[1]: {1, 2, 3}

Array2:
[2]: {1, 2, 3}

Array3:
[2]: {1, 2, 3}

UPDATE: Is there any built-in function to do this? Or is the FOR loop the unique solution?

Answer 1

I would do the following:

• Create a class that takes a row in the array and implements hashcode and equals.
• Insert each row (wrapped in the above class) in the first two arrays into two HashMaps
• for each row in the last array, determine if they exist in the HashMaps

Edit #2, realized the mapping would need to be reversed.

private Map<MyClass, Integer> map(int[] array){
  Map<MyClass, Integer> arrayMap = new HashMap<>();
  for (int i; i<array.length; i++)
        arrayMap.put(new MyClass(array[i]), i);
}

private mySearch(){
   int[] array1, array2, array3;

   Map<MyClass, Integer> map1 = map(array1);
   Map<MyClass, Integer> map2 = map(array2);

   for (int i=0; i<array3.lenght; i++){
      MyClass row = new MyClass(array3[i]);
      Integer array1Row = map1.get(row);
      Integer array2Row = map2.get(row);

      if (array1Row != null && array2Row != null){
         // Matching rows found
      }
   }
}