Run a loop from 3 to n Bottom up approach to fill the array
for ( int i = 3 ; i <= n ; i ++ ) ways [ i % 3 ] = ways [ ( i - 1 ) % 3 ] + ways [ ( i - 2 ) % 3 ] + ways [ ( i - 3 ) % 3 ] ; return ways [ n % 3 ] ; }