How to write a Surprisingly Happy Ending without making it feel cheap?

So I\'m planning to write a story that suffers from Cerberus Syndrome, and has dark plot twists near the end that change the entire mood. The problem is, I don\'t like bad endings. I don\'t want my characters\' efforts, and possibly deaths, to be in vain and I don\'t want the audience leaving feeling depressed or cheated, so I definitely want something that turns the tables. But another problem is, I don\'t want the solution to come out of the blue and call it a day. I want to make it satisfying for both the characters and the audience.

Is there any tricks and tips on how to do this?

Establishing props and plot points early on, reusing them across the whole length of the story and maintaining internal consistency.

Sorry that the answer is so short, but the question is vague, the issue is complex and giving more detailed guides requires to have specific pieces of writing at hand. But making the audience not feel cheated, avoiding Ass Pulls and Deus Ex Machinae is always about foreshadowing and internal consistency.