Longest substring computational complexity

Question

I was wondering about my apparently code for an MIT online edx class practice problem vs the provided answer code.

The assignment in question from the class was

Answer 1

Their answer code is more efficient because it does not repeatedly iterate through subsequences. Given the subsequence ABCDE, your code separately processes BCDE, CDE, and DE in successive iterations even though they cannot be longest.

Therefore, the worst-case runtime of your answer is O(N^2) vs. O(N) for theirs. Yes, this is related to having a nested for loop which is not present in their answer.