LeetCode 3 Longest Substring Without Repeating Characters - Why Naive Fails

Check all substrings. For each substring, scan to verify all characters are unique.

There are $O(n^2)$ substrings. Checking each for uniqueness takes $O(n)$. Total: $O(n^3)$.

For $n = 5 \times 10^4$, that's about $10^{14}$ operations. Far too slow.