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)
CONSTRAINTS
-a will be between 1 and 1,000,000,000, inclusive.
-b will be between a and 1,000,000,000, inclusive.
EXAMPLES
0)
1
10
Returns: 2
There are only two lucky numbers among the first ten positive integers.
1)
11
20
Returns: 0
But there are none among the next ten.
2)
74
77
Returns: 2
These two numbers are lucky. There are no additional lucky numbers between them.
3)
1000000
5000000
Returns: 64
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