This is a problem that can be found in McDowell's "Cracking the Coding Interview" workbook. When the interviewers gave me the problem, I let them know that I had seen this problem before (which is generally recommended. Shows honesty and some interviewers can tell when you're faking working through a problem you've practically memorized). They said to go ahead and solve it anyway, which was fine since it had been awhile since I had touched the problem. Before diving into the code, I compared whether it would be easier to add the numbers from front-to-back or vice versa and voiced my thoughts aloud. I proceeded to solve the problem (can't remember which approach I wound up taking). When I had to take time to think, I pseudocoded and diagrammed the portion I was working on.