Often to save some time, I would like we to use n = len(s) in my local function.
I am curious about which call is faster or they are the same? 

```
while i < len(s):
  # do something

```

vs

```
while i < n:
  # do something

```

There should not be too much difference, but using len(s), we need to reach s first, then call s.**length**. This is O(1) + O(1). But using n, it is O(1). I assume so.