text: Precomputation of the countDP prefix sum of the matrix
code: countDP [ 0 ] [ 0 ] = matrix [ 0 ] [ 0 ] ; for ( let i = 1 ; i < R ; i ++ ) countDP [ i ] [ 0 ] = countDP [ i - 1 ] [ 0 ] + matrix [ i ] [ 0 ] ; for ( let j = 1 ; j < C ; j ++ ) countDP [ 0 ] [ j ] = countDP [ 0 ] [ j - 1 ] + matrix [ 0 ] [ j ] ; for ( let i = 1 ; i < R ; i ++ ) for ( let j = 1 ; j < C ; j ++ ) countDP [ i ] [ j ] = matrix [ i ] [ j ] + countDP [ i - 1 ] [ j ] + countDP [ i ] [ j - 1 ] - countDP [ i - 1 ] [ j - 1 ] ;