Print an NxM matrix with nw-se diagonals

import java.io.*;

class myCode
{
    public static void main (String[] args) throws java.lang.Exception
    {
        int n = 3;
        int m = 4;
        int arr[][] = new int[n][m];
        arr[0] = new int[] { 1, 2, 3, 4 };
        arr[1] = new int[] { 5, 6, 7, 8 };
        arr[2] = new int[] { 9, 10, 11, 12 };
        
        //ist=>i_start
        //jst=>j_start
        int i=n-1,j=0,ist,jst,count=0;
        while(count<n*m){
            ist=i;jst=j;
            while(i<n && j<m){
                System.out.print(arr[i][j]+" ");
                count++;
                i++;j++;
            }
            System.out.println();
            i=(ist-1)>=0?ist-1:ist;
            j=(jst==0 && (ist-1)>=0)?0:jst+1;
        }
    }
}