A friend who works at Facebook referred me, and an HR representative contacted me about a week later. We scheduled a 45-minute technical phone interview for a week and a half out.

For the actual interview, I began with a couple of general questions about myself and Facebook before moving on to the technical questions. There were two programming questions.

For the first question, I first described my algorithm, then discussed its time and space complexity, and finally implemented it. My interviewer had no comment on my answer, and we simply moved on.

I took the same basic approach to the second question: I described my algorithm, discussed its space/time complexity, and then implemented it. However, after discussing the complexity of my potential solution, my interviewer asked if I saw any drawbacks to my algorithm. I mentioned that it used a lot of extra space, and he asked if I could think of a more space-efficient solution. I pondered it for a while but unfortunately blanked out. He then suggested I proceed with implementing my algorithm.

The last five minutes or so of the 45-minute interview were dedicated to my questions. I asked a few, and the interview concluded.

I received a generic rejection email three days later.

Why are you interested in Facebook?

What is your favorite feature of Facebook?

As follow-ups, what would you change about this feature, and what could be a drawback of this change?

Given "data", "from", and "to" fields, replace all occurrences of the characters in the "from" field within the "data" field with their counterparts in the "to" field.

Example: Input: Data: "Hello World" From: "lod" To: "xpf" Output: "Hexxp Wprxf"

Create an iterator class for a tree. The iterator should traverse the tree using in-order traversal. Implement the constructor and next functions.