# java-归并排序

```import java.util.Arrays;

public class MergeSort {

public static void main(String[] args) {

int[] a={20,1,3,8,5,9,4,25};
mergeSort(a,0,a.length-1);
System.out.println(Arrays.toString(a));
}

public static void mergeSort(int[] a,int first,int last){
int len=a.length;
if(first>=last)return;
if(first<last){
//int mid=(first+last)/2;
//to avoid overflow,you should do it like this:
int mid=(first&last)+((first^last)>>1);
int[] temp=new int[len];
mergeSort(a,first,mid);
mergeSort(a,mid+1,last);
mergeTwoArray(temp,a,first,mid,last);
}
}

public static void mergeTwoArray(int[] temp,int[] a,int first,int mid,int last){
int i=first;
int j=mid+1;
int k=0;
while(i<=mid&&j<=last){
if(a[i]<a[j]){
temp[k]=a[i];
i++;
}else{
temp[k]=a[j];
j++;
}
k++;
}
while(i<=mid){
temp[k++]=a[i++];
}
while(j<=last){
temp[k++]=a[j++];
}
//System.arraycopy(src, srcPos, dest, destPos, length);
//at first,i write it like this :
//System.arraycopy(temp, 0, a, 0, temp.length);
//I can't get it right until I do it like this:
System.arraycopy(temp, 0, a, first, last-first+1);
//System.arraycopy(temp, 0, a, first, k);//it also works
}
}

```

java-归并排序

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