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

or, in one pass and without the overhead of string concatenation:

length, start, stop, i = len(s), 0, 0, 0

while i < length:

    j = i+1

    while j < length and s[j] >= s[j-1]:
        j += 1

    if j - i > stop - start:
        start, stop = i, j

    i = j

print(s[start:stop])