Use user submissions for computing problem difficulty only for problems that have >= 3 submissions.

Author: MipsaPatel

private/scripts/extras/problem_difficulty.py

@@ -80,6 +80,7 @@
 FROM problem_difficulty
 WHERE problem_id IN (%(pids)s)
 GROUP BY problem_id
+HAVING count(*) >= 3
 """ % ({"pids": ",".join(pids)})
 
 res = db.executesql(sql_query)