Build the PrimitiveMedium
Longest Substring Without Repeating Characters
Write `length_of_longest_substring(s)` returning the length of the longest run of characters with no repeats.
length_of_longest_substring("abcabcbb") -> 3 ("abc")
length_of_longest_substring("bbbbb") -> 1
Keep a map from character to its last index. On a repeat, move the left edge to one past that index — but only ever forwards, never backwards, or a stale entry from outside the window will drag it back.
What to expect: A timer starts when you begin. Edit the starter code, run it against the test suite as many times as you like, then finish when you're done. The reference solution and interviewer follow-up questions unlock only after you finish.