Dustforce

First off, if you'd rather just watch a video, here is one made by youtube user Calistus, which covers the locations of 50 apples:

However, there are more than 50 total apples in the game, so you don't have to get all of them. I've made a list of all the ones I know of, so you can decide which ones to go for and which ones to skip. A lot of their locations would be very hard to describe because of how messy some of the levels are, so only a few of them have notes letting you know where they are. Just explore and you'll find the others without too many issues. Also, once you've swept an apple once, feel free to just exit out of the level, no need to complete it or anything. And yes, it does have to be 50 different apples.

So without further ado, my own list:

Forest:

18 total apples
Downhill - 2 apples (first is on a big tower in the upper left of the level)
Firefly Forest - 1 apple
Valley - 2 apples
Dusk Run - 1 apple on a big tower on the left side of the map, go through underground tunnels or above
Overgrown Temple - 1 apple (see video)
Grass Cave - 3 apples (see video for apple #1)
Ascent - 1 apple (drop down right from the top)
Summit - 3 apples (drop left then right from start, and dash through a tiny hole in a spike pit later on, see video for apple #3)
Ancient Garden - 1 apple
Ruins - 2 apples
Wild Den - No apples
Night Temple - 1 apple (see video)

City:

16 total apples
Apartments - 3 apples (jump far left from the first platform after slope for the first one)
Warehouse - 1 apple (on roof)
Park - 3 apples (second apple on roof after first slide part, use the green character, then another on the higher roof on the same building)
Construction Site - 1 apple (in an alcove high on the left, use an X+Y attack to refill your jump when you run out)
Forgotten Tunnel - 2 apples
Basement - 1 apple
Scaffolding - 1 apple
Rooftops - 1 apple
Clocktower - 1 apple
Hideout - 1 apple
Alleyway - No apples
Concrete Temple - 1

Mansion:

12 total apples
Courtyard - 1 apple
Cliffside Caves - 2 apples
Library - 1 apple
Caverns - 2 apples
Archive - 1 apple
Knight Hall - 1 apple
Store Room - No apples
Ramparts - 1 apple
Tower - 1 apple
Ghost Parapets - 1 apple
Observatory - No apples
Moon Temple - 1 apple

Laboratory:

12 total apples
Security - 1 apple
Research - 3 apples (all in the same room!)
Server Room - 1 apple (slide under the big spiky things on the right, don't jump through)
Vats - 1 apple
Access - 1 apple
Power Room - 1 apple
Wiring - No apples
Core Temple - 1 apple
Dome - 1 apple
Backup Shift - 1 apple
Abyss - 1 apple

If you have any extra locations to add, if you can't find any specific apple, or if you have any other questions, feel free to leave a comment and I'll get back to you. Good luck! :)