Question

I have a 2D Array and I would like to find an easier way to manipulate my code so that it will find if there is a duplicate in the column and easier way then what I have below:

for (int i=0; i < array.length; i++) {
    for (int j=0; j < array.length; j++) {
        for (int k=1; k < array.length; k++){
            if (array[j+k][i] == array[j][i]) {
               if (array[j][i] != 0) {
                return true;
               }
            }
        }
    } 
  }  
return false;

EDIT: KINDLY POINTED OUT THE ABOVE ^^ WON’T WORK EITHER AS IT WILL THROW AN OUT OF BOUNDS EXCEPTION

This way has too many loops and I am sure there must be an easier way to find duplicates rather than going through this massive looping process.

This is for a square 2D array, ie. an array with rows = columns.

If so, how can this new way work – and how can I manipulate it to work to find duplicate values in the rows as well.

Thanks for the help.

Answer 1

you can use HashSet to store all already encountered elements. should be something like this:

static boolean noDupes(int[][] array) {
    for (int i=0; i < array.length; i++) {
        HashSet<Integer> set = new HashSet<Integer>();
        for (int j=0; j < array.length; j++) {
            if (set.contains(array[j][i])) return false;
            set.add(array[j][i]);
        }
    }
    return true;
}

this solution is O(length^2) = O(n) where n is the matrix total size. I think it is ideal in terms of big O, because you need to check all elements.