US immigration officials forced a Nigerian software engineer to complete a written test on binary search trees to prove his computer knowledge.

Customs and Border Protection officers, took Celestine Omin, 28, into a room for further checks after he arrived at New York’s JFK Airport last week, 24 hours after he set off.

He told them he worked for Andela, a tech start-up with offices in New York, Lagos, Nairobi and San Francisco, which claims to take "the most talented developers on the African continent" and link them with potential US employers.

The firm has offices in Kenya’s capital Nairobi, San Francisco, New York and the Nigerian city of Lagos, which was visited by Facebook founder Mark Zuckerberg.

One of the officers then presented him with a piece of paper and a pen and told to answer these two questions to prove he is actually a software engineer:

"Write a function to check if a Binary Search Tree is balanced."

"What is an abstract class, and why do you need it?"

In computer science, binary search trees are a particular type of data structure that store items such as numbers or names.

Omin told Linkedin that he thought the questions could have multiple answers and looked to him like someone with no technical background Googled something like: “Questions to ask a software engineer.”

After spending about 10 minutes working on them, he handed in his answers only to be told they were wrong.

As time passed, he said that he expected to be sent home to Nigeria, only for the official to let him go.

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“He said, ‘Look, I am going to let you go, but you don’t look convincing to me,’” Omin said. “I didn’t say anything back. I just walked out.”

It later emerged that the officers had phoned Andela to verify his story.