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Java/Maths/VampireNumber.java at master · ailvcheng/Java · GitHub
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VampireNumber.java
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package
Maths
;
import
java
.
util
.
ArrayList
;
import
java
.
util
.
Collections
;
/**
* n number theory, a vampire number (or true vampire number) is a composite natural number with an
* even number of digits, that can be factored into two natural numbers each with half as many
* digits as the original number and not both with trailing zeroes, where the two factors contain
* precisely all the digits of the original number, in any order, counting multiplicity. The first
* vampire number is 1260 = 21 × 60. *
*
* <p>* link: https://en.wikipedia.org/wiki/Vampire_number *
*
* <p>
*/
public
class
VampireNumber
{
public
static
void
main
(
String
[]
args
) {
test
(
10
,
1000
);
}
static
void
test
(
int
startValue
,
int
stopValue
) {
int
countofRes
=
1
;
StringBuilder
res
=
new
StringBuilder
();
for
(
int
i
=
startValue
;
i
<=
stopValue
;
i
++) {
for
(
int
j
=
i
;
j
<=
stopValue
;
j
++) {
// System.out.println(i+ " "+ j);
if
(
isVampireNumber
(
i
,
j
,
true
)) {
countofRes
++;
res
.
append
(
""
+
countofRes
+
": = ( "
+
i
+
","
+
j
+
" = "
+
i
*
j
+
")"
+
"
\n
"
);
}
}
}
System
.
out
.
println
(
res
);
}
static
boolean
isVampireNumber
(
int
a
,
int
b
,
boolean
noPseudoVamireNumbers
) {
// this is for pseudoVampireNumbers pseudovampire number need not be of length n/2 digits for
// example
// 126 = 6 x 21
if
(
noPseudoVamireNumbers
) {
if
(
a
*
10
<=
b
||
b
*
10
<=
a
) {
return
false
;
}
}
String
mulDigits
=
splitIntoDigits
(
a
*
b
,
0
);
String
faktorDigits
=
splitIntoDigits
(
a
,
b
);
return
mulDigits
.
equals
(
faktorDigits
);
}
// methode to Split the numbers to Digits
static
String
splitIntoDigits
(
int
num
,
int
num2
) {
StringBuilder
res
=
new
StringBuilder
();
ArrayList
<
Integer
>
digits
=
new
ArrayList
<>();
while
(
num
>
0
) {
digits
.
add
(
num
%
10
);
num
/=
10
;
}
while
(
num2
>
0
) {
digits
.
add
(
num2
%
10
);
num2
/=
10
;
}
Collections
.
sort
(
digits
);
for
(
int
i
:
digits
) {
res
.
append
(
i
);
}
return
res
.
toString
();
}
}
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