NewPipe is a lightweight YouTube front-end for Android. It doesn't need the YouTube-API or any Google Play Services, as NewPipe only parses the YouTube website in order to gain the information it needs.
Get it today, for free! Get it from below link
https://f-droid.org/packages/org.schabi.newpipe/
Its simply lightweight and i'm using works fine for those who want to watch videos with specific resolution and better small memory smartphones .Its completely opensource so battery optimization is too good and there is a option to download videos while playing with required video resolution.I think this post will be helpful to all people if it does plz vote and follow me.