fix

Signed-off-by: Wei Jin <wjin.cn@gmail.com>
little fix

Signed-off-by: Wei Jin <wjin.cn@gmail.com>
fix

Signed-off-by: Wei Jin <wjin.cn@gmail.com>
one more solution

Signed-off-by: Wei Jin <wjin.cn@gmail.com>
neat solution

Signed-off-by: Wei Jin <wjin.cn@gmail.com>
add comment

Signed-off-by: Wei Jin <wjin.cn@gmail.com>
neat solution

Signed-off-by: Wei Jin <wjin.cn@gmail.com>
recursive solution

Signed-off-by: Wei Jin <wjin.cn@gmail.com>
add morris post order traverse

Signed-off-by: Wei Jin <wjin.cn@gmail.com>