How to capitalize every other letter in a string efficiently in java?

I'm currently working on a problem in code hunt level 6.02 which asks me to capitalize every other letter in a String. I have tried doing it with toCharArray + StringBuilder in for loops. It works, but it's not good enough. I still can't get the perfect score for the problem. I'm running out of ideas. Any help will be greatly appreciated.

Note: This is my first post on stack overflow. So if I miss anything or ask question in a wrong way. Pls feel free to point it out for me. Thx.