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Java/java-combinations/Combination.java at master · taipham/Java · GitHub
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Combination.java
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java-combinations
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Combination.java
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package
com
.
hmkcode
.
baloot
;
public
class
Combination
{
public
static
void
main
(
String
[]
args
){
Object
[]
elements
=
new
Object
[] {
'A'
,
'B'
,
'C'
,
'D'
,
'E'
,
'F'
,
'G'
,
'H'
};
combination
(
elements
,
7
);
}
public
static
void
combination
(
Object
[]
elements
,
int
K
){
// get the length of the array
// e.g. for {'A','B','C','D'} => N = 4
int
N
=
elements
.
length
;
if
(
K
>
N
){
System
.
out
.
println
(
"Invalid input, K > N"
);
return
;
}
// calculate the possible combinations
// e.g. c(4,2)
c
(
N
,
K
);
// get the combination by index
// e.g. 01 --> AB , 23 --> CD
int
combination
[] =
new
int
[
K
];
// position of current index
// if (r = 1) r*
// index ==> 0 | 1 | 2
// element ==> A | B | C
int
r
=
0
;
int
index
=
0
;
while
(
r
>=
0
){
// possible indexes for 1st position "r=0" are "0,1,2" --> "A,B,C"
// possible indexes for 2nd position "r=1" are "1,2,3" --> "B,C,D"
// for r = 0 ==> index < (4+ (0 - 2)) = 2
if
(
index
<= (
N
+ (
r
-
K
))){
combination
[
r
] =
index
;
// if we are at the last position print and increase the index
if
(
r
==
K
-
1
){
//do something with the combination e.g. add to list or print
print
(
combination
,
elements
);
index
++;
}
else
{
// select index for next position
index
=
combination
[
r
]+
1
;
r
++;
}
}
else
{
r
--;
if
(
r
>
0
)
index
=
combination
[
r
]+
1
;
else
index
=
combination
[
0
]+
1
;
}
}
}
public
static
int
c
(
int
n
,
int
r
){
int
nf
=
fact
(
n
);
int
rf
=
fact
(
r
);
int
nrf
=
fact
(
n
-
r
);
int
npr
=
nf
/
nrf
;
int
ncr
=
npr
/
rf
;
System
.
out
.
println
(
"C("
+
n
+
","
+
r
+
") = "
+
ncr
);
return
ncr
;
}
public
static
int
fact
(
int
n
)
{
if
(
n
==
0
)
return
1
;
else
return
n
*
fact
(
n
-
1
);
}
public
static
void
print
(
int
[]
combination
,
Object
[]
elements
){
String
output
=
""
;
for
(
int
z
=
0
;
z
<
combination
.
length
;
z
++){
output
+=
elements
[
combination
[
z
]];
}
System
.
out
.
println
(
output
);
}
}
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