File: reverse-only-letters/solution.py

Date: 2026-06-06

Time: 18:53

Purpose

This file implements LeetCode problem 917 - Reverse Only Letters. It provides a Solution class with a method that reverses only the English alphabetic characters in a string while leaving all non-letter characters (digits, punctuation, spaces) in their original positions.

Note: the method is misnamed numrescueboats — this is a copy-paste artifact from a different problem. The docstring and implementation correctly solve "Reverse Only Letters."

Key Components

Solution.numrescueboats(self, s: str) -> str

Contract: Takes a string s, returns a new string where all isalpha() characters appear in reverse order but non-alpha characters remain at their original indices.

Example: "a-bC-dEf-ghIj" → "j-Ih-gfE-dCba"

Patterns

Two-pointer convergence: Classic technique where left starts at 0 and right at len-1. Both pointers move inward, skipping non-letter characters. When both point at letters, the values are swapped. This avoids needing to extract letters, reverse them, and re-insert — doing the work in a single pass over the array.

The string is converted to a list first since Python strings are immutable, then joined back at the end.

Dependencies

Imports: None — uses only Python builtins (str.isalpha, list, str.join).

Imported by: reverse-only-letters/test_solution.py (and hundreds of other test files that appear to share a common test harness importing all solutions).

Flow

1. Convert s to a mutable list of characters

2. Initialize two pointers at the string boundaries

3. Loop while left < right:

• If chars[left] is not alphabetic, advance left
• Else if chars[right] is not alphabetic, retreat right
• Else swap the two characters and advance both pointers

4. Join and return the result

The elif/else structure means exactly one pointer moves (or both move on swap) per iteration, guaranteeing O(n) time with O(n) space for the list copy.

Invariants

• Pointer ordering: left < right is checked before every operation — the loop terminates when pointers meet or cross.
• Non-letter stability: Non-alpha characters are never moved; only alpha characters participate in swaps.
• Single-pass: Each index is visited at most once by either pointer.

Error Handling

None. The method trusts that s is a valid string. Empty strings and single-character strings work correctly — the while left < right guard handles both cases by skipping the loop entirely.

Topics to Explore

• [file] reverse-only-letters/test_solution.py — See what edge cases are tested (empty string, all-alpha, no-alpha, single char)
• [function] reverse-vowels-of-a-string/solution.py — Same two-pointer-with-filter pattern applied to vowels instead of all letters
• [file] reverse-only-letters/review.md — May document the method name mismatch and other review findings
• [general] two-pointer-convergence-pattern — How this repo applies the converging two-pointer idiom across different string/array problems

Beliefs

• method-name-mismatch — numrescueboats is an incorrect method name for the Reverse Only Letters problem; the implementation solves problem 917, not the rescue boats problem
• non-alpha-positional-invariant — Non-alphabetic characters in the input string are guaranteed to remain at their original indices in the output
• two-pointer-linear-time — The algorithm runs in O(n) time because each pointer advances monotonically inward and they collectively visit each index at most once
• no-external-dependencies — The solution uses only Python built-in operations (str.isalpha, list, str.join) with zero imports