text: Loop to find prefix sum of the given matrix
code: for ( int r = R - 1 ; r >= 0 ; r -- ) { for ( int c = C - 1 ; c >= 0 ; c -- ) { preSum [ r ] = arr [ r ] ; if ( r + 1 < R ) preSum [ r ] += preSum [ r + 1 ] ; if ( c + 1 < C ) preSum [ r ] += preSum [ r ] ; if ( r + 1 < R && c + 1 < C ) preSum [ r ] -= preSum [ r + 1 ] ; } }