Do walking and swimming help fighting the inflammation in the kneecap and help regenerate the cartilage ( i'm 21 years old , no rehumatoid)?

Knee pain: Those are both low impact exercises that will help with keeping you in motion. I would also increase diet, consider glucosamine as some people have found improvement in cartilage. That being said ligaments should be properly evaluated by orthopedic.

No: Inflammation can be the result of overuse. While swimming is a good exercise for those with joint inflammation from more gravity heavy exercises like walking, the motion can still continue to aggravate the inflammation. Any inflammation that doesn't resolve with 3 weeks of rest needs orthopedic evaluation.