Imperfect solution to compute an absolute difference of 2 diagonals from a square matrix ?

See more:

C++

algorithm

I'm trying to do an exercise from Hackerrank to compute an absolute difference of 2 diagonals from a square matrix. I have a code, but not perfect since it passes only 3/8 sample cases.
Below, is a sample test case, my code can solve this case, unfortunately I can't get access to different test cases including those that my code failed to solve because I don't have a premium account. Here's basically what the task is :

Dimension matrix : 3 

11 2   4    | Diagonal 1 : 11 5 -12 -- sum_d1 =  4
 4 5   6    | Diagonal 2 :  4 5  10 -- sum_d2 = 19
10 8 -12    | Abs difference diagonals : abs(sum_d1-sum_d2) = 15

Sample Input

3          //user input -- dimension of matrix square (n)
11 2   4   //actual input of matrix elements, rows based on dimension (n)
4  5   6   //for each row, input each element separated by a white space
10 8 -12

Sample Output

15 

What I have tried:

Here's my code, I'm planning to treat this square matrix as a 1D array

#include <iostream>
#include <cmath>

using namespace std;

int main(){
    int n;
    cin >> n; // square matrix of what dimension ?
    int size = n*n;
    
    int arr[size]; 
    int iter = 0;
    
    //filling in array, input separated by white space for each row (n)
    for (int i = 0; i < n; i++){
        cin >> arr[iter] >> arr[iter+=1] >> arr[iter+=1];
        iter++;
    }

    int diag1    = 0, diag2 = 0;
    int sum      = 0;
    int step1    = 0, step2 = 0;

    //finding diagonal elements and sum them
    for (int j = 0 ; j < n; j++){
        diag1 += arr[step1];
        // cout << "\nElements Diagonal 1 = " << arr[step1];
        step1 += (n+1);
        step2 += (n-1);
        diag2 += arr[step2];
        // cout << "\nElements Diagonal 2 = " << arr[step2];
    }

    sum = abs(diag1-diag2);
    cout << sum << endl;
    
    return 0;
}

Running :
3
11 2   4
 4 5   6
10 8 -12
15       (correct answer)

/** commented out in my code, to check if I got the right elements
Elements Diagonal 1 = 11
Elements Diagonal 2 = 4
Elements Diagonal 1 = 5
Elements Diagonal 2 = 5
Elements Diagonal 1 = -12
Elements Diagonal 2 = 10
------------------------- **/ 

I am aware that there are solutions online with various approaches. However, I want to try to solve this myself and want to know what did I miss and where did my code go wrong ? As I mentioned earlier, my code passes 3/8 sample cases, and I can't check failed cases because it requires premium account. Therefore, instead of a new approach, if possible please help me understand where / how did my code would fail. Thank you.


[UPDATE]
Solution was to modify the array insertion block to be :

//filling in array with input separated by space for each row (n)
    for (int i = 0; i < n; i++){	
        for ( int j = n; j > 0; j--){
        cin >> arr[iter];
        iter++;
	}
  }

Thank you @CPallini for pointed out the mistake in my code.