I answered using a loop that built the reverse string one character at a time, but since it was for a mobile developer position, they then asked for something more memory-efficient. I wasn't able to come up with an answer, but they said you could have a character placeholder and just swap characters one at a time. (Upon further thinking after the interview, I'm not sure if this works in Java, which is what I was thinking of, where I was taught strings were immutable.)