Decode Ways / Alphanumeric Combination

Classic decode-ways DP: given a digit string, count the number of ways to decode it using the mapping A=1, B=2, …, Z=26. Appears verbatim on Goldman's OA bank. Requirements Input: a non-empty digit string s. Mapping: "1" → A, "2" → B, …, "26" → Z. Output: total number of distinct decodings of s. Constraints: leading 0s and isolated 0s are invalid (e.g. "06" cannot decode); two-digit codes are valid only in [10, 26]. public static int alphanumbericCombination(String s) Notes Standard 1-D DP. dp[i] = number of ways to decode the first i characters. Transition: dp[i] += dp[i-1] if s[i-1] is in 1..9; dp[i] += dp[i-2] if s[i-2..i-1] is in 10..26. O(n) time, O(1) space if you collapse the array to two scalars. Edge cases that the visible OA tests do not always catch: "0" alone should return 0; "10", "20" should return 1; "27" returns 1 (only single-digit decoding works). Equivalent to the canonical LeetCode "Decode Ways" problem; a * wildcard follow-up exists in the LC variant but has not been reported at Goldman. Preparation Implement once with a full dp[] array, then refactor to two rolling scalars on the second pass — Goldman interviewers occasionally ask for the space-optimized form. Drill the leading-zero / standalone-zero edge cases by hand before submitting. LC 91 (Decode Ways) is the canonical equivalent; LC 639 adds the * wildcard for further practice.