final sclass SparseIntArray2D implements IIntArray2D {
  int nrows, ncols;
  new HashMap<Pt, Int> entries;
  
  *() {}
  *(int *nrows, int *ncols) {}
  
  public int nrows() { ret nrows; }
  public int ncols() { ret ncols; }

  /** Returns A(i, j). */
  public int get(int i, int j) {
    ret or0(entries.get(Pt(i, j));
  }

  /** Sets A(i, j). */
  public int set(int i, int j, int x) {
    if (x == 0)
      entries.remove(Pt(i, j));
    else
      entries.put(Pt(i, j), x);
    ret x;
  }
}