A LinkedIn recruiter contacted me a few months ago (around November 2014, I believe) about a Software Engineering position.

After the initial non-technical phone screen, I was scheduled for my first technical phone interview. I decided to take two months to prepare and refresh my knowledge of Algorithms and Data Structures.

Finally, I finished my first interview very disappointed (with myself). The interview question was so easy that I could have written the solution in less than 15 minutes, yet I spent almost 45, thanks to my nerves. The interviewer was polite, explained things well, and called on time; I have no complaints about that.

Write a function to determine if a string is a number without using any built-in functions.