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JavaArithmetic/src/basic/commonAlgorithm.java at master · wangguofei/JavaArithmetic · GitHub
wangguofei
JavaArithmetic
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JavaArithmetic
/
src
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basic
/
commonAlgorithm.java
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119 lines (98 loc) · 2.7 KB
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JavaArithmetic
/
src
/
basic
/
commonAlgorithm.java
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119 lines (98 loc) · 2.7 KB
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public
class
commonAlgorithm
{
/**
* 判断一个数是否为素数
*
* @param num
* @return
*/
private
static
boolean
isPrime
(
int
num
) {
if
(
num
<=
1
)
return
false
;
if
(
num
==
2
)
return
true
;
if
(
num
%
2
==
0
)
return
false
;
for
(
int
x
=
3
;
x
*
x
<=
num
;
x
+=
2
)
if
(
num
%
x
==
0
)
return
false
;
return
true
;
}
// 计算arr[l...n)这个区间内所有数字的和
private
static
int
sum
(
int
[]
arr
,
int
l
) {
if
(
l
==
arr
.
length
)
return
0
;
return
arr
[
l
] +
sum
(
arr
,
l
+
1
);
}
/**
* 交换数组元素
*
* @param arr
* @param i
* @param j
*/
private
static
void
swap
(
Object
[]
arr
,
int
i
,
int
j
) {
if
(
i
<
0
||
i
>=
arr
.
length
)
throw
new
IllegalArgumentException
(
"i is out of bound."
);
if
(
j
<
0
||
j
>=
arr
.
length
)
throw
new
IllegalArgumentException
(
"j is out of bound."
);
Object
temp
=
arr
[
i
];
arr
[
i
] =
arr
[
j
];
arr
[
j
] =
temp
;
}
/**
* 求n的和
*
* @param n
* @return
*/
private
static
int
sum
(
int
n
) {
if
(
n
<
0
)
throw
new
IllegalArgumentException
(
"n should be greater or equal to zero."
);
int
ret
=
0
;
for
(
int
i
=
0
;
i
<=
n
;
i
++)
ret
+=
i
;
return
ret
;
}
/**
* 翻转数组
*
* @param arr
*/
private
static
void
reverse
(
Object
[]
arr
) {
int
n
=
arr
.
length
;
for
(
int
i
=
0
;
i
<
n
/
2
;
i
++)
swap
(
arr
,
i
,
n
-
1
-
i
);
}
/**
* 选择排序
*
* @param arr
* @param n
*/
private
static
void
selectionSort
(
Comparable
[]
arr
,
int
n
) {
for
(
int
i
=
0
;
i
<
n
;
i
++) {
int
minIndex
=
i
;
for
(
int
j
=
i
+
1
;
j
<
n
;
j
++)
if
(
arr
[
j
].
compareTo
(
arr
[
minIndex
]) <
0
)
minIndex
=
j
;
swap
(
arr
,
i
,
minIndex
);
}
}
/**
* 二分查找
*
* @param arr
* @param n
* @param target
* @return
*/
public
static
int
binarySearch
(
Comparable
[]
arr
,
int
n
,
Comparable
target
) {
int
l
=
0
,
r
=
n
;
// 在[l...r)的范围里寻找target
while
(
l
<
r
) {
// 当 l == r 时, 区间[l...r)是一个无效区间
int
mid
=
l
+ (
r
-
l
) /
2
;
if
(
arr
[
mid
].
compareTo
(
target
) ==
0
)
return
mid
;
if
(
target
.
compareTo
(
arr
[
mid
]) >
0
)
l
=
mid
+
1
;
// target在[mid+1...r)中; [l...mid]一定没有target
else
// target < arr[mid]
r
=
mid
;
// target在[l...mid)中; [mid...r)一定没有target
}
return
-
1
;
}
}
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