Decode ways DP: how do you handle 0 and 06 edge cases?

Question presented to candidate: "A message of letters A to Z was encoded as numbers, A = 1, B = 2, up to Z = 26, and the numbers were written next to each other with no separators. Given the digit string, count how many ways it can be decoded. Pay special attention to zeros — what do you do with a string like '06' or '100'? Code it in Python."

What a strong answer should cover:

• The state: dp[i] = number of ways to decode the first i digits, with dp[0] = 1 (the empty prefix has one decoding: nothing).
• The two moves: the last letter used one digit (allowed only if that digit is not 0, add dp[i-1]) or two digits (allowed only if they form 10 to 26, add dp[i-2]).
• The zero rules: a 0 can never stand alone, so it must be the second digit of 10 or 20; 06 is not a letter because a two-digit code cannot start with 0; 30, 00 or a leading 0 make the whole answer 0.
• O(n) time and O(1) space with two rolling variables, compared with the exponential plain recursion (the all-1 string grows like Fibonacci).
• Python details: comparing single characters (s[i - 1] == "2" and s[i] <= "6") instead of int(s[i-1:i+1]) avoids building a slice each step, and int("06") returns 6, which is exactly the bug the question is fishing for.

Clarifying questions expected:

• Can the string contain zeros, and can it start with one?
• What should an empty string return?
• Can the answer be large — should it be returned modulo something?

Code / implementation expected: Yes — a one-pass Python DP with two rolling variables, tested on "06", "10", "100" and "226".