数组搜索（search）之线性搜索与二分法搜索

线性搜索代码比较简单，也通俗易懂，就是挨个遍历，查找数组中与key相同的元素，若查找不到，则可以返回-1（惯例，自定义），其效率为O(n).

二分法搜索代码稍稍复杂，有一个很重要的前提是数组必须是有序的。定义一个索引low（表征搜索索引下限），high（表征搜索索引上限），mid（即索引中间值）。

当key与mid位置的元素相同时，直接将mid返回，而当key大于mid位置的元素，即key若存在一定存在于后半段时，这是我们可以将mid+1赋值给low,重新计算mid。此时还是比较key与mid位置的元素，若key与mid位置的元素相同时，直接将mid返回，而当key小于mid位置的元素时，我们将mid-1赋值给high，重新计算mid。这样反复进行下去，直到low>high，此时表明数组里面没有key相同的元素。

线性搜索代码

import java.util.Random;
import java.util.Arrays;

public class LinearArray {
private int[] data;
private static final Random generator = new Random();

public LinearArray(int size){
data = new int[size];
for (int i =0; i<size;i++){
data[i] = generator.nextInt(90) + 10;
}
}
public int linearSearch(int searchKey){
for (int index = 0; index<data.length; index++){
if (data[index] == searchKey)
return index;
}
return -1;
}
public String toString(){
return Arrays.toString(data);
}
}

测试类

import java.util.Scanner;
public class LinearSearchTest {

public static void main(String[] args) {
// TODO Auto-generated method stub
Scanner input = new Scanner(System.in);
int searchInt, position;
LinearArray searchArray = new LinearArray(10);
System.out.println(searchArray+"\n");

System.out.print("Please enter an integer value(-1 to quit):");
searchInt = input.nextInt();
while(searchInt != -1){
position = searchArray.linearSearch(searchInt);
if (position == -1)
System.out.println("The integer "+ searchInt+" was not found.\n");
else
System.out.println("The integer "+searchInt+" was found in position "
+position+"\n");
System.out.print("please enter an integer value(-1 to quit):");
searchInt = input.nextInt();
}
}

}

二分法搜索代码

import java.util.Random;
import java.util.Arrays;
public class BinarySearch {
private int[] data;
private static final Random generator = new Random();

public BinarySearch(int size){
data = new int[size];
for (int i = 0; i< data.length;i++)
data[i] = 10 + generator.nextInt(90);
Arrays.sort(data);
}

public int binarySearch(int searchElement){
int low = 0;
int high = data.length-1;
int mid = (low+high+1)/2;
int location = -1;

do{
if (searchElement == data[mid])
location = mid;
else if (searchElement <data[mid])
high = mid -1;
else
low = mid + 1;
mid = (low+high+1)/2;

} while((low<=high)&&(location ==-1));
return location;

}
public String toString(){
return Arrays.toString(data);
}
}

测试类

import java.util.Scanner;

public class BinarySearchTest {

public static void main(String[] args) {
// TODO Auto-generated method stub
Scanner input = new Scanner(System.in);
int searchInt, position;
BinarySearch searchArray = new BinarySearch(10);
System.out.println(searchArray.toString()+"\n");

System.out.print("Please enter an integer value(-1 to quit):");
searchInt = input.nextInt();
while(searchInt != -1){
position = searchArray.binarySearch(searchInt);
if (position == -1)
System.out.println("The integer "+ searchInt+" was not found.\n");
else
System.out.println("The integer "+searchInt+" was found in position "
+position+"\n");
System.out.print("please enter an integer value(-1 to quit):");
searchInt = input.nextInt();
}
}

}