[英]Runtime error in my mergesort implementation
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];
}
}
這是行不通的。 編譯后,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)
如何解決此問題?
您有兩個稱為a
數組:
靜態的mergesort.a
:
public static int a[];
和main中的局部變量:
int a[]=new int[]{64,21,33,70,12,85,44,99,36,108};
當您使用a
在merge
,使用靜態成員,這是從來沒有初始化。
您要初始化mergesort.a
,應該已經編寫了
a = new int[]{64,21,33,70,12,85,44,99,36,108};
但是,這會使merge
變得很奇怪,因為它將有work = a
作為參數,並引用了靜態mergesort.a
,該靜態mergesort.a
之后將是同一數組。
困惑? 我也是... :)
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