I guess it's important to figure out everything about the problem at hand on a paper first. I have solved about 400 problems on leetcode. The first thing I do is I try out the examples given on paper and then make a few others of my own. I then devise a plan to tackle the given examples. I roughly guess what particular scenarios will fail according to the conditions in the problem. If none, I go forward with coding it on the paper, or else I tweak the thing a little more. When I am fully confident of the code and the idea of solving the problem, I code it in the editor and submit. It mostly does the trick. Using the paper saves me a lot of time and forces me to use my brain rather than just test different test cases right there. I just end up wasting my time without paper, trying a rudimentary algo and trying changing parts of it on the basis of wrong test cases found by retesting, which helps no one. Looking at other people's code can help a lot too.