File: decrypt-string-from-alphabet-to-integer-mapping/solution.py

Date: 2026-06-06

Time: 16:12

Decrypt String from Alphabet to Integer Mapping

Purpose

This file implements LeetCode problem #1309 — decoding a numeric string where digits 1–9 map to letters a–i, and two-digit sequences 10#–26# map to j–z. It's a self-contained solution + test module following the repo's standard layout.

Key Components

Solution.sortItems(self, s: str) -> str — Misleadingly named (should be freqAlphabets per the LeetCode problem). Walks the string left-to-right, consuming either 3 characters (when a # follows at position i+2) or 1 character, converting each numeric token to its corresponding letter via chr(ord("a") + num - 1).

TestSolution — Eight test cases covering single digits, double digits with #, mixed sequences, boundary between 9/10, and the full a–z alphabet.

Patterns

• Greedy lookahead: The while loop peeks two positions ahead (s[i + 2] == "#") to decide whether to consume a 2-digit-plus-hash token or a single digit. The lookahead is guarded by a bounds check (i + 2 < len(s)), so it never overruns.
• Character arithmetic: chr(ord("a") + num - 1) converts 1-based numeric values to lowercase letters — standard for alphabet-mapping problems.
• Combined solution + test file: The repo convention puts Solution and TestSolution in the same solution.py, runnable via unittest.main().

Dependencies

Imports: Only unittest from the standard library — no external dependencies.

Imported by: The "Imported By" list in the prompt is misleading — those are unrelated test files across the repo. The actual consumer is decrypt-string-from-alphabet-to-integer-mapping/test_solution.py, which imports Solution from this module.

Flow

1. Initialize an empty result list and index i = 0.

2. At each position, check if s[i+2] exists and equals "#".

• Yes: parse s[i:i+2] as a two-digit integer (10–26), advance i by 3.
• No: parse s[i] as a single-digit integer (1–9), advance i by 1.

3. Convert the integer to a letter and append to result.

4. Join and return.

Invariants

• The input is assumed to be a valid encoded string: digits 1–9 standalone, digits 10–26 always followed by #. No validation is performed.
• The lookahead prioritizes the 3-character ## token — this is correct because a # at position i+2 unambiguously signals a double-digit encoding. Single digits can never be followed by # in a valid input.
• num is always in [1, 26] for valid inputs, so the output is always lowercase a–z.

Error Handling

None. Invalid inputs (non-digit characters, out-of-range numbers, malformed # placement) will either raise ValueError from int() or produce garbage output silently. This is typical for LeetCode solutions where inputs are guaranteed valid.

Notable Issue

The method is named sortItems, which is the name for LeetCode #1203 (Sort Items by Groups Respecting Dependencies). The correct LeetCode method name for problem #1309 is freqAlphabets. This won't affect functionality but makes the code confusing to navigate.

Topics to Explore

• [file] decrypt-string-from-alphabet-to-integer-mapping/test_solution.py — Separate test file that imports this Solution; check if it duplicates or extends the inline tests
• [file] decrypt-string-from-alphabet-to-integer-mapping/review.md — Code review notes that may flag the method naming issue
• [general] greedy-lookahead-parsing — Compare with 1-bit-and-2-bit-characters/solution.py which uses the same peek-ahead-to-decide-token-width pattern
• [function] decode-the-message/solution.py:Solution — Another alphabet-mapping problem; compare the character arithmetic approach

Beliefs

• decrypt-method-misnamed — Solution.sortItems should be named freqAlphabets; the current name belongs to LeetCode #1203, not #1309
• hash-lookahead-greedy-correct — The 3-char token (XX#) is always checked before the 1-char token, which is the only correct parse order for this encoding
• no-input-validation — The solution assumes all inputs are valid encoded strings and will raise ValueError or produce wrong output on malformed input
• chr-arithmetic-maps-1-to-a — chr(ord("a") + num - 1) maps integer 1 to 'a' and integer 26 to 'z', covering the full lowercase alphabet