排序-mergesort

void merge(int l,int r)
{
    if(l>=r) return ;
    int mid=(l+r)>>1;
    merge(l,mid);
    merge(mid+1,r);
    int i=l,j=mid+1,k=0;
    while(i<=mid&&j<=r){
        if(a[i]<=a[j]) b[k++]=a[i++];
        else b[k++]=a[j++];
    }
    while(i<=mid) b[k++]=a[i++];
    while(j<=r) b[k++]=a[j++];
    for(int ii=0;ii<k;ii++) a[ii+l]=b[ii];
     
    return ;
}