**In java**

    public static void main(String[] args) {
        for(int i = 0 ,k=1; i>=0; i+=(k==1)?1:-1){
            for(int c = 65, d = 1; c>=65;c+=(d==1)?1:-1){
                System.out.print((char)c);
                d = (d!=-1 && c < 65+i)?1:-1;
            }
            k = (k!=-1 && i < 25)?1:-1;
            System.out.println();
        }
    }