Notepad++ is one of most used application on windows environment, can be used for various languages like C, C++, CSS, HTML, javscript, PHP ...., this software is used widely for web development, thats why if someone switching to Open-source and adopt Linux then we start searching for its alternative. If you want to use Notepad++ on Linux then simply install wine (allow Linux users to run Windows applications on their Linux) . Now you can easily install Wine. But if you want alternatives of Notepad++ then you can install these software :
Geany (My Personal Favourite) :
Light weight, stable and fast, Supports syntax highlighting for several languages, auto-indent, tabs, etc. Can be used for languages like Java, C, C++ ....
Sublime Text is a sophisticated text editor for code, markup and prose. You'll love the slick user interface, extraordinary features and amazing performance. Pleanty of plugins available that makes your life simple but it's not free.
Notepad++ is ported to linux under the name Notepadqq, It's being actively developed here. But I will not recommed as it is not enough stable for usages but you can give it a try.
Kate / Kwrite :
Kate also known as advanced editor comes defaultwith KDE desktop .kwrite is also built on kate. So you can use one of them. First Kate is good replacement of Notepad++ ,almost has all feature, lightweight , see list of some feature
Komodo Edit :
and 64bit version is also available.