遞歸搜索錯誤

Question

運行到一個愚蠢的錯誤，我只是沒有看到它。我一直在看這一段時間，並沒有看到我錯過了什麼。我遞歸搜索一個數組的特定目標編號，但一旦我到達元素[7]它開始返回-1。謝謝你看看傢伙/女士們！

public static void main(String[] args) 
    { 
     int[] a = {1,25,2,6,4,3,23,30,32,14,11,8}; 
     Arrays.sort(a); 
     int target = a[7]; 
     int first = a[0]; 
     int last = a.length; 
     for(int i=0;i<a.length;i++) 
     { 
      System.out.print(" "+a[i]); 
    } 
     System.out.println("\n"+binarySearch(target,first,last,a)); 
    } 
    public static int binarySearch(int target,int first, int last, int[] a) 
    { 
     int result; 
     if(first>last) 
      return -1; 
     else 
     { 
      int mid = (first+last)/2; 
      if(target == mid) 
       result = mid; 
      else if(target<a[mid]) 
       result = binarySearch(target,first,last-1,a); 
      else 
       result = binarySearch(target,mid+1,last,a); 

     } 
     return result; 
    } 

Answer 1

在幾個地方，您無法準確區分數組索引中的值和索引本身。

此：a[i]在第i個元素

這得到值：i是一個簡單的指標，我

考慮到這一點，這裏是你的代碼的固定版本。看到我的代碼中的註釋爲我修正了一些具體錯誤：

public static void main(String[] args) 
{ 
    int[] a = {1,25,2,6,4,3,23,30,32,14,11,8}; 
    Arrays.sort(a); 
    int target = a[7]; 
//here you want the index of the first location to search, not the value in that index 
//so you use 0 instead of a[0] 
    int first = 0; 
//the last element index is length-1, not length, since arrays are 0-based 
    int last = a.length - 1;  
    for(int i=0;i<a.length;i++) 
    { 
     System.out.print(" "+a[i]); 
    } 
    System.out.println("\n"+binarySearch(target,first,last,a)); 
} 

public static int binarySearch(int target,int first, int last, int[] a) 
{ 
    int result; 
    if(first>last) 
     return -1; 
    else 
    { 
     int mid = (first+last)/2; 
//here you need to check if the target is equal to the value at the index mid 
//before you were checking if the target was equal to the index, which was never true 
     if(target == a[mid]) 
//you want to return the value at the target, not the index of the target 
//so use a[mid] not mid 
      result = a[mid]; 
     else if(target<a[mid]) 
//here you want to search from first to mid-1 
//before you were searching from first to last-1, which is not correct binary search 
      result = binarySearch(target,first,mid - 1,a); 
     else 
      result = binarySearch(target,mid + 1,last,a); 

    } 
    return result; 
}