使用Java的数组排序和二进制搜索

Question

我被要求对数组进行排序和搜索。数组的排序很简单，我的代码也很有效，但每当我尝试调用二进制搜索方法时，它对数组中的第一个元素有效，但结果是“-1”

我的完整代码如下：

 public static void main(String[] args) {

    int[] array = new int[5];
    array[0] = 50;
    array[1] = 40;
    array[2] = 10;
    array[3] = 20;
    array[4] = 100;

sort(array, (array.length - 1));

      for (int x = 0; x < array.length; x++) {
        System.out.println(" " + array[x]);
    }
    System.out.println("");
    System.out.println("Binary search (R): " + rBsearch(array, 0, (array.length), 20));
}
    public static void sort(int[] a, int last) {
    if (last > 0) {
        int max = findMax(a, last);
        swap(a, last, max);
        sort(a, last - 1);
    }
}

public static int rBsearch(int[] L, int low, int high, int k) {


    int mid = (low + high) / 2;

    if (low > high) {
        return -1;
    } else if (L[mid] == k) {
        return mid;
    } else if (L[mid] < k) {
        return rBsearch(L, k, mid + 1, high);
    } else {
        return rBsearch(L, k, low, mid - 1);
    }
 }

public static int findMax(int[] arr, int last) {

    int max = 0;
    for (int i = 0; i <= last; i++) {
        if (arr[i] > arr[max]) {
            max = i;
        }
    }
    return max;
    }

public static void swap(int[] arr, int last, int max) {
    int temp = arr[last];
    arr[last] = arr[max];
    arr[max] = temp;
}

Answer 1

1. 从用户获取数组
2. 使用Java的内置函数对数组进行排序

3. 然后使用二进制搜索来搜索元素

       import java.lang.reflect.Array;
       import java.util.Arrays;
       import java.util.Scanner;
   
       class BinarySearch
       {
          public static void main(String args[])
          {
             int array[];
   
             Scanner input = new Scanner(System.in);
             System.out.println("Enter number of elements:");
            int Size_Of_Array = input.nextInt(); 
   
   
             array = new int[Size_Of_Array];
   
             System.out.println("Enter " + Size_Of_Array + " integers");
   
             for (int counter = 0; counter < Size_Of_Array; counter++)
                 array[counter] = input.nextInt();
   
             Arrays.sort(array);
             System.out.println("Sorting Array is :-");
             for (int counter = 0; counter < Size_Of_Array; counter++)
                 System.out.println(array[counter]);
   
             System.out.println("Enter the search value:");
             int  Searching_item = input.nextInt();
   
             int First_Index=0;
             int Last_Index=Size_Of_Array-1;
             int Middle_Index=(First_Index+Last_Index)/2;
   
             while(First_Index <= Last_Index)
             {
                 if(array[Middle_Index] < Searching_item)
                 {
                     First_Index=Middle_Index+1;
                 }
                 else if ( array[Middle_Index] == Searching_item ) 
                 {
                   System.out.println(Searching_item + " found at location " + (Middle_Index + 1) + ".");
                   break;
                 }
                 else
                 {
                     Last_Index = Middle_Index - 1;
                 }
                 Middle_Index = (First_Index + Last_Index)/2;
   
                 if ( First_Index > Last_Index )
                 {
                     System.out.println(Searching_item + " is not found.\n");
                 }
             }
           }
       }
   

   Result of BinarySearch

Answer 2

最简单的方法是： 将数组转换为列表：Arrays.asList(array)

对于排序：Collections#sort

搜索：Collections#binarySearch

见this

Answer 3

你搞错了二进制搜索间隔

public static int rBsearch(int[] L, int low, int high, int k) {


    int mid = (low + high) / 2;

    if (low > high) {
        return -1;
    } else if (L[mid] == k) {
        return L[mid];
    } else if (L[mid] < k) {
        return rBsearch(L, mid + 1, high, k);
    } else {
        return rBsearch(L, low, mid - 1, k);
    }
 }

Answer 4

在以下几行中调用rBsearch方法时出错 而不是

else if (L[mid] < k) {
        return rBsearch(L, k, mid + 1, high); 
    } else {
        return rBsearch(L, k, low, mid - 1);
    }

你应该使用

else if (L[mid] < k) {
            return rBsearch(L,  mid + 1, high,k); //the order of the parameters
        } else {
            return rBsearch(L, low, mid - 1,k);
        }