Base cases ( process first three elements )
if ( n >= 1 ) sum [ 0 ] = arr [ 0 ] ; if ( n >= 2 ) sum [ 1 ] = arr [ 0 ] + arr [ 1 ] ; if ( n > 2 ) sum [ 2 ] = max ( sum [ 1 ] , max ( arr [ 1 ] + arr [ 2 ] , arr [ 0 ] + arr [ 2 ] ) ) ;