I only had two phone interviews.

The first one was fairly easy:

• What is the time complexity of quicksort?
• Inserting into a binary tree?

The second one was a coding problem.

You are given an n by n array filled with '0's, and you have to set randomly X number of '1's in that array.

Looping and generating X random numbers doesn't work as you might get the same random number. Collisions will become more likely, especially as X gets closer to n*n. In such cases, you'll loop for a very long time before filling all the '1's.

All positions must have the same probability of being selected, and the array might be huge.