Celestine Omin tweeted:

I was just asked to balance a Binary Search Tree by JFK's airport immigration. Welcome to America.

BoingBoing explains:

Celestine Omin is a Nigerian software engineer who works for Mark Zuckerberg and Priscilla Chan's company Andela, founded to give talented African coders an entree into the leading American tech firms; this week, he flew to the USA on a B1/B2 visa to meet with the company, but he found himself detained at the border.

The CBP guards who detained Omin after his 24-hour flight were skeptical that he was a real software engineer. They apparently googled "quizzes to give to software engineers" and told him to answer ten questions (e.g. "Write a function to check if a Binary Search Tree is balanced" and "What is an abstract class, and why do you need it?") to gain entry into the country.

OMG