我的mergesort實現中的運行時錯誤

Question

我寫了下面的mergesort代碼。

public class mergesort { 

    public static int a[]; 

    public static void merges (int work[], int low, int high) { 
     if (low==high) 
      return; 
     else { 
      int mid = (low+high)/2; 
      merges(work,low,mid); 
      merges(work,mid+1,high); 
      merge(work,low,mid+1,high); 
     } 
    } 

    public static void main (String[] args) { 
     int a[] = new int[] {64, 21, 33, 70, 12, 85, 44, 99, 36, 108}; 
     merges(a,0,a.length-1); 
     for (int i=0; i<a.length; i++) { 
      System.out.println(a[i]); 
     } 
    } 


    public static void merge (int work[], int low, int high, int upper) { 
     int j = 0; 
     int l = low; 
     int mid = high-1; 
     int n = upper-l+1; 
     while (low<=mid && high<=upper) 
      if (a[low] < a[high]) 
       work[j++] = a[low++]; 
      else 
       work[j++] = a[high++]; 
     while (low <= mid) 
      work[j++]=a[low++]; 
     while (high <= upper) 
      work[j++] = a[high++]; 
     for (j=0;j<n;j++) 
      a[l+j]=work[j]; 
    } 
} 

它不起作用。編譯後出現此錯誤：

java.lang.NullPointerException 
    at mergesort.merge(mergesort.java:45) 
    at mergesort.merges(mergesort.java:12) 
    at mergesort.merges(mergesort.java:10) 
    at mergesort.merges(mergesort.java:10) 
    at mergesort.merges(mergesort.java:10) 
    at mergesort.main(mergesort.java:27) 

如何修復此問題？

Answer 1

你有兩個數組稱爲a：

一個靜態的，mergesort.a：

public static int a[]; 

和局部變量主：

int a[]=new int[]{64,21,33,70,12,85,44,99,36,108}; 

當您在merge使用a，您使用靜態成員，它從未初始化。
它你想初始化mergesort.a，你應該寫

a = new int[]{64,21,33,70,12,85,44,99,36,108}; 

然而，這將使merge奇怪，因爲這將有work = a作爲參數，並參照靜態mergesort.a，那麼這將是相同的陣列。

困惑？所以我... :)