Loop to find the number the number of ways to reach Nth stair
for ( int i = 4 ; i <= n ; i ++ ) { includes_3 [ i ] = includes_3 [ i - 1 ] + includes_3 [ i - 2 ] + not_includes_3 [ i - 3 ] ; not_includes_3 [ i ] = not_includes_3 [ i - 1 ] + not_includes_3 [ i - 2 ] ; } return includes_3 [ n ] ; }