合并排序

 public static void  mergeSort(int [] a,int l,int r){
        if(l==r)
            return ;
        int mid=l+(r-l)/2;
        mergeSort(a,l,mid);
        mergeSort(a,mid+1,r);
        merge(a,l,mid,r);
    }
    public static void merge(int [] a,int l,int m,int r){
        int i=l;
        int p1=l;
        int p2=m+1;
        int [] tmp=new int[r-1+1];
        while(p1<=m&&p2<=r){
            tmp[i++]=a[p1]<a[p2]?a[p1++]:a[p2++];
        }
        while(p1<=m){
            tmp[i++]=a[p1++];
        }
        while(p2<=r){
            tmp[i++]=a[p2++];
        }
        for(int j=0;j<tmp.length;j++){
            a[l+j]=tmp[j];
        }
    }