Java 使用比较器的二进制搜索
我正在努力让它工作。我需要编写一个函子,它将使用二进制搜索算法来查找长度在12到15个单位之间的阶梯 以下是二进制搜索:Java 使用比较器的二进制搜索,java,comparator,binary-search,functor,Java,Comparator,Binary Search,Functor,我正在努力让它工作。我需要编写一个函子,它将使用二进制搜索算法来查找长度在12到15个单位之间的阶梯 以下是二进制搜索: public static <AnyType> int binarySearch(GenericSimpleArrayList<AnyType> a, AnyType x, Comparator<? super AnyType> cmp) { int low = 0; int high = a.size() - 1;
public static <AnyType> int binarySearch(GenericSimpleArrayList<AnyType> a, AnyType x, Comparator<? super AnyType> cmp) {
int low = 0;
int high = a.size() - 1;
int mid;
while (low <= high) {
mid = (low + high) / 2;
if (cmp.compare(a.get(mid), x) < 0) {
low = mid + 1;
} else if (cmp.compare(a.get(mid), x) > 0) {
high = mid - 1;
} else {
return mid;
}
}
return NOT_FOUND; // NOT_FOUND = -1
}
publicstaticintbinarysearch(genericsimplearraylista,AnyType x,Comparator尝试以下方法:
public class FindLadder implements Comparator<Ladder>{
@Override
public int compare(Ladder lhs, Ladder rhs) {
if(lhs.length() < rhs.length() && lhs.length() > 12) // Suppose rhs.length() is 15
{
return 0;
}
if(lhs.length() < 12) {
return -1;
}
else {
return 1;
}
}
}
公共类FindLadder实现比较器{
@凌驾
公共整数比较(左梯形图、右梯形图){
if(lhs.length()12)//假设rhs.length()为15
{
返回0;
}
如果(左侧长度()<12){
返回-1;
}
否则{
返回1;
}
}
}
用x=15调用binarySearch()
。就像LibraryComparator.binarySearch(l,新梯形图(15),新FindLadder());
我硬编码了12
。没有其他方法。您显然希望/需要实现您自己的二进制搜索,但还是让我参考内置方法
从以下文件的javadoc:
返回搜索键的索引(如果它包含在列表中);否则,((插入点)-1)
。插入点定义为将键插入列表的点:大于键的第一个元素的索引,或list.size()
如果列表中的所有元素都小于指定的键。请注意,这保证了当且仅当找到键时返回值>=0
这里的关键是插入点,或者换句话说,返回的索引指向第一个元素,即>=搜索值,或者size()
(如果不存在这样的元素)。没有NOT\u FOUND
返回值
由于您需要介于12
和15
之间的值,请搜索12
,然后通过破解Collections.binarySearch验证是否找到了值,以及该值是否为短版本
该框架具有,您应该使用它们
内置的binarySearch
函数始终将pivot元素作为第二个参数提供给比较器。这是未记录的行为,但我们可以使用以下比较器利用这一点:
public static class FindLadderInterval implements Comparator<Ladder> {
public final int min, max;
public FindLadderInterval(int min, int max) {
this.min = min;
this.max = max;
}
@Override
public int compare(Ladder lhs, Ladder rhs) {
// ignore rhs
int length = lhs.length();
return length < this.min ? -1 : length > this.max ? 1 : 0;
}
}
工作示例:
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class Main2 {
public static class Ladder {
private final int _length;
public Ladder(int length) {
this._length = length;
}
public int length() {
return this._length;
}
@Override
public String toString() {
return "Ladder(" + this._length + ")";
}
}
public static class FindLadderInterval implements Comparator<Ladder> {
public final int min, max;
public FindLadderInterval(int min, int max) {
this.min = min;
this.max = max;
}
@Override
public int compare(Ladder lhs, Ladder rhs) {
// ignore rhs
int length = lhs.length();
return length < this.min ? -1 : length > this.max ? 1 : 0;
}
}
public static void main(String[] args) {
List<Ladder> list = new ArrayList<Ladder>();
list.add(new Ladder(1));
list.add(new Ladder(2));
list.add(new Ladder(6));
list.add(new Ladder(13));
list.add(new Ladder(17));
list.add(new Ladder(21));
int index = Collections.binarySearch(list, null,
new FindLadderInterval(12, 15));
System.out.println("index: " + index);
System.out.println("ladder: " + list.get(index));
}
}
要使用内置的Collections.binarySearch
或您的函数,您需要提供一个具有代表性的元素,这在按字符串长度排序时非常困难。要查找长度为15的字符串,必须提供长度为15的字符串。这就是为什么我更喜欢python样式的排序,它使用key函数或选择器。基本上,您不需要比较,只需要映射到一个可比较的值。例如,从String
到Integer
的映射,如s->s.length()
。这样就可以实现像这样的甜美函数(lambdas使它变得漂亮):
确保binarySearch算法的前提条件:
- 项目必须提前按升序存储
- 您必须提供具有要查找的功能的项
那就试试这个
public int compare(Ladder lhs, Ladder rhs) {
boolean isLhsInRange = lhs.length() >= 12 && lhs.length() <= 15;
boolean isRhsInRange = rhs.length() >= 12 && rhs.length() <= 15;
if(isLhsInRange && isRhsInRange) return 0;
else return lhs.length() - rhs.length();
}
public int比较(梯形图左侧、右侧){
布尔值isLhsInRange=lhs.length()>=12&&lhs.length()=12&&rhs.length()好的,我有另一个类似的functor类做同样的事情。我如何调用binarySearch方法,同时也应用我需要的条件?我不确定这是否正确。可能的返回值是-1
,0
,1
@SarahvdByl我不能得到你说的东西?@Saud例如,在我的主方法中,我curre使用LibraryComparator.binarySearch(l,15,new Length());
这不起作用。我不知道如何指定我正在搜索12到15个单位长的阶梯。@SarahvdByl您的binarySearch
方法只有一个参数x
,它可以与其他阶梯进行比较。如果您想获得特定范围内的阶梯,需要两个参数x
和y
。任何需要在A
和B
之间查找内容的搜索方法,其本质上都需要同时知道A
和B
,即上下边界。您的搜索方法只需要1x
,那么您如何分辨这两个数字12
和15
嗯,这就是我正在努力的地方。我看不到任何可能的方法。我可以硬编码它,因为我知道我要寻找的梯子的位置,但显然可以在函子中做一些事情来避免这种情况。@SarahvdByl我编辑了我的答案。
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class Main2 {
public static class Ladder {
private final int _length;
public Ladder(int length) {
this._length = length;
}
public int length() {
return this._length;
}
@Override
public String toString() {
return "Ladder(" + this._length + ")";
}
}
public static class FindLadderInterval implements Comparator<Ladder> {
public final int min, max;
public FindLadderInterval(int min, int max) {
this.min = min;
this.max = max;
}
@Override
public int compare(Ladder lhs, Ladder rhs) {
// ignore rhs
int length = lhs.length();
return length < this.min ? -1 : length > this.max ? 1 : 0;
}
}
public static void main(String[] args) {
List<Ladder> list = new ArrayList<Ladder>();
list.add(new Ladder(1));
list.add(new Ladder(2));
list.add(new Ladder(6));
list.add(new Ladder(13));
list.add(new Ladder(17));
list.add(new Ladder(21));
int index = Collections.binarySearch(list, null,
new FindLadderInterval(12, 15));
System.out.println("index: " + index);
System.out.println("ladder: " + list.get(index));
}
}
public static <T, R extends Comparable<? super R>> int intervalBinarySearchBy(
List<T> list, R min, R max, Function<? super T, ? extends R> selector) {
int idx = binarySearchBy(list, max, selector);
if (idx >= 0) return idx;
// Collections.binarySearch returns the insertion index binary
// negated if the element was not found
idx = ~idx;
return (idx < list.size()
&& min.compareTo(selector.apply(list.get(idx))) <= 0) ? idx : -1;
}
List<Person> list = getPersons();
Person youngest = minBy(list, p -> p.getAge());
Person tallest = maxBy(list, p -> p.getHeight());
Person person42 = findBy(list, 42, p -> p.getAge());
sortBy(list, p -> p.getAge());
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.function.Function;
public class Main {
public static class Collections2 {
/**
* Mimics Collections.binarySearch
*
* @param list
* @param pivotKey
* @param selector
* @return
*/
public static <T, R extends Comparable<? super R>> int binarySearchBy(
List<T> list, R pivotKey,
Function<? super T, ? extends R> selector) {
int low = 0;
int high = list.size() - 1;
while (low <= high) {
int mid = (low + high) >>> 1;
int ord = selector.apply(list.get(mid)).compareTo(pivotKey);
if (ord < 0) {
low = mid + 1;
} else if (ord > 0) {
high = mid - 1;
} else {
return mid;
}
}
return ~high; // bitwise negated insertion point /* -(a+1) == ~a */
}
/**
* Finds the index of the last element in the interval, or returns -1 if
* no such element was found.
*
* @param list
* @param min
* @param max
* @param selector
* @return
*/
public static <T, R extends Comparable<? super R>> int intervalBinarySearchBy(
List<T> list, R min, R max, Function<? super T, ? extends R> selector) {
int idx = binarySearchBy(list, max, selector);
if (idx >= 0) return idx;
// Collections.binarySearch returns the insertion index binary
// negated if the element was not found
idx = ~idx;
return (idx < list.size()
&& min.compareTo(selector.apply(list.get(idx))) <= 0) ? idx : -1;
}
public static <T, R extends Comparable<? super R> > Comparator<T> comparatorBy(
Function<? super T, ? extends R> selector) {
return (a, b) -> selector.apply(a).compareTo(selector.apply(b));
}
}
public static Function<Ladder, Integer> LENGTH_OF = a -> a.length();
public static class Ladder {
private final int _length;
public Ladder(int length) {
this._length = length;
}
public int length() {
return this._length;
}
@Override
public String toString() {
return "Ladder(" + this._length + ")";
}
}
public static void main(String[] args) {
List<Ladder> list = new ArrayList<Ladder>();
list.add(new Ladder(5));
list.add(new Ladder(9));
list.add(new Ladder(14));
list.add(new Ladder(7));
list.add(new Ladder(22));
list.add(new Ladder(23));
list.add(new Ladder(11));
list.add(new Ladder(9));
Collections.sort(list, Collections2.comparatorBy(LENGTH_OF));
int i = 0;
for (Ladder s : list) {
System.out.println("" + (i++) + ": " + s);
}
int foundIdx = Collections2.intervalBinarySearchBy(list, 12, 15,
LENGTH_OF);
System.out.println("Index: " + foundIdx);
System.out.println(list.get(foundIdx));
}
}
public int compare(Ladder lhs, Ladder rhs) {
boolean isLhsInRange = lhs.length() >= 12 && lhs.length() <= 15;
boolean isRhsInRange = rhs.length() >= 12 && rhs.length() <= 15;
if(isLhsInRange && isRhsInRange) return 0;
else return lhs.length() - rhs.length();
}