Currently ranking is shown in alphabetical order, i.e if two users have solved the same number of problems, they will be ranked alphabetically in the list. I think it will be better if they were ranked on the basis of time. The user who solves a challenge first should be ranked before another who solved the same after him.
Furthermore, if we assign different number of points to the challenges, again the ranking is done in alphabetical order and on the basis of number of challenges solved. But, the ranking should be on the basis of number of points earned.