How many different natural numbers can be formed with the numbers $$1$$, $$2$$, $$3$$?（without using the same number two times like $$33$$） 
Overseas Competition->Knowledge Point->Counting Modules->Questions Involving Enumeration->Dictionary Ordering
Classify the number first by the digits and then enumerate．  One-digit number：$$1$$、$$2$$、$$3$$，  Two-digit number：$$12$$、$$13$$、$$21$$、$$23$$、$$31$$、$$32$$；  Three-digit number：$$123$$、$$132$$、$$213$$、$$231$$、$$312$$、$$321$$；  So totally $$3+6+6=15$$ different natural numbers can be formed． 