TheLuckyNumbers | Topcoder Archive

Problem Statement

John thinks 4 and 7 are lucky digits, and all other digits are not lucky. A lucky number is a number that contains only lucky digits in decimal notation.

You are given ints a and b. Return the number of lucky numbers between a and b, inclusive.

Definition

Class:: TheLuckyNumbers
Method:: count
Parameters:: int, int
Returns:: int
Method signature:: int count(int a, int b)
(be sure your method is public)

Constraints

a will be between 1 and 1,000,000,000, inclusive.
b will be between a and 1,000,000,000, inclusive.

Examples

```
1
```
```
10
```
```
Returns: 2
```
There are only two lucky numbers among the first ten positive integers.
```
11
```
```
20
```
```
Returns: 0
```
But there are none among the next ten.
```
74
```
```
77
```
```
Returns: 2
```
These two numbers are lucky. There are no additional lucky numbers between them.
```
1000000
```
```
5000000
```
```
Returns: 64
```

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