int tmp[n];
int i=l,j=mid+1;
int k=0;
while(i<=mid && j<=r){
if(a[i]<=a[j]) tmp[k++] = a[i++];
else tmp[k++] = a[j++];
}
while(i<=mid) tmp[k++] = a[i++];
while(j<=r) tmp[k++] = a[j++];
for(int i=l,j=0;i<=r;i++,j++) a[i]=tmp[j];