PROJECT EULER · #0698
123 Numbers
We define 123-numbers as follows:
- 1 is the smallest 123-number.
- When written in base 10 the only digits that can be present are "1", "2" and "3" and if present the number of times they each occur is also a 123-number.
So 2 is a 123-number, since it consists of one digit "2" and 1 is a 123-number. Therefore, 33 is a 123-number as well since it consists of two digits "3" and 2 is a 123-number.
On the other hand, 1111 is not a 123-number, since it contains 4 digits "1" and 4 is not a 123-number.
In ascending order, the first 123-numbers are:
Let
Find
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