text: Count total number of 1 s in the sub square considered
code: int count = countDP [ x2 ] [ y2 ] ; if ( x1 > 0 ) count -= countDP [ x1 - 1 ] [ y2 ] ; if ( y1 > 0 ) count -= countDP [ x2 ] [ y1 - 1 ] ; if ( x1 > 0 && y1 > 0 ) count += countDP [ x1 - 1 ] [ y1 - 1 ] ;