Your protestations in a previous thread notwithstanding, this is clearly homework. What gave you away? Err, how about quoting the question verbatim (necessarily in a writing style differing from your own).
in reply to Least common ancestor function implemented in perl
If you're doing homework, we'd love to see what you've tried already. I suspect you've made no good-faith attempt at all. It's simply unfair to palm your work off onto PM (where someone will eventually give you an answer).
Whether your 'homework' is for a high school, college/university, or a self-study program... the same considerations apply to effort and pay-off in the long run.
Ethically, I also believe you should put "Homework: " in the title and <blockquote></blockquote> the question to clearly denote the text as having been written by another... (if for no ethical reason) to satisfy copyright fair-use exemptions.