File: split-a-string-in-balanced-strings/solution.py

Date: 2026-06-06

Time: 19:14

Purpose

This file implements LeetCode problem 1221 — Split a String in Balanced Strings. It solves the problem of finding the maximum number of balanced substrings (substrings with equal counts of 'L' and 'R') that a balanced string can be split into. The file is self-contained: it defines the solution function and its unit tests in one module.

Key Components

findspecialinteger(s: str) -> int

Despite its misleading name (likely a copy-paste artifact from a template — the function has nothing to do with "special integers"), this is the core solver. Its contract:

• Input: A string s containing only 'L' and 'R', guaranteed to be balanced overall (equal total L and R counts).
• Output: The maximum number of non-overlapping balanced substrings s can be split into.
• Strategy: Greedy — split as early as possible. Every time the running balance returns to zero, that's one more balanced substring.

TestFindSpecialInteger

Six test cases covering the LeetCode examples plus edge cases: minimal input ("RL"), alternating characters, and nested structure ("RRLLRRLL").

Patterns

Greedy counter pattern. The solution uses a single integer balance as a stand-in for tracking 'R' vs 'L' counts. 'R' increments, 'L' decrements. When balance hits zero, both characters have appeared equally — that's a split point. This is the canonical O(n) greedy approach for this problem; no stack or auxiliary data structure needed.

Self-contained test module. The file bundles unittest.TestCase alongside the solution, runnable via python -m unittest or python solution.py directly through the if _name_ guard.

Dependencies

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

Imported by: The testsolution.py in this same directory, plus hundreds of testsolution.py files across other problem directories. This is almost certainly a dependency-graph artifact — those other test files likely import from their *own* solution.py, not this one. The Imported By list reflects a tool that resolved import solution ambiguously across the flat directory structure.

Flow

1. Initialize balance = 0 and result = 0.

2. Iterate character by character through s.

3. For each 'R', add 1 to balance; for each 'L', subtract 1.

4. Whenever balance == 0, increment result — we've found a balanced substring.

5. Return result.

For "RLRRLLRLRL": the balance trace is [1,0,1,2,1,0,1,0,1,0], hitting zero at indices 1, 5, 7, 9 → returns 4.

Invariants

• The algorithm assumes s is balanced overall (equal L and R). If it isn't, the final balance won't be zero, but the function still returns a count — it just won't account for the trailing unbalanced portion.
• The greedy "split at every zero-crossing" is provably optimal for maximizing the number of balanced splits, because deferring a split can never create more splits later.
• balance is always in the range [-len(s), len(s)].

Error Handling

None. The function trusts its input — no validation that s contains only 'L'/'R', no check that the string is balanced. Consistent with LeetCode solution conventions where inputs are guaranteed by the problem constraints.

Topics to Explore

• [file] split-a-string-in-balanced-strings/plan.md — The problem analysis and approach rationale written before implementing the solution
• [file] split-a-string-in-balanced-strings/review.md — Post-implementation review, likely flags the misleading function name
• [general] greedy-balance-counter — This same balance-counter technique appears in parentheses problems (valid parentheses, max nesting depth) — worth comparing how the zero-crossing test adapts across problems
• [function] split-a-string-in-balanced-strings/test_solution.py:* — The separate test file may have additional or different test cases beyond what's inline here
• [general] function-naming-conventions — findspecialinteger is clearly wrong for this problem; checking whether other solutions share this template artifact

Beliefs

• greedy-split-is-optimal — Splitting at every zero-crossing of the balance counter produces the maximum number of balanced substrings; no other strategy yields more splits.
• function-name-mismatch — findspecialinteger does not match the problem it solves; it likely originates from a code-generation template that wasn't updated per-problem.
• r-positive-l-negative-convention — The solution assigns +1 to 'R' and -1 to 'L'; swapping this convention would produce identical results since only zero-crossings matter.
• no-input-validation — The function performs no checks on its input; passing characters other than 'L'/'R' silently produces wrong results rather than raising an error.