Abstract / Summary

Implement a function that converts all uppercase letters in a given string to lowercase and returns the resulting string.

Note

The full problem description, including helpful examples, can be found on LeetCode.

This problem is ranked as

and has an acceptance rate of about 84% (600k+ submissions).

Typical patterns to solve this problem:

• String

Every single line of code was written by me, 100% from scratch, between August 2023 and August 2024.
However, two years later I set up this website and used various AI tools to generate most of the walkthrough / explanations below.

See this dedicated page for more details.

Overview

The provided implementation defines a Solution class with a single public method toLowerCase. The method receives a mutable copy of a std::string and converts every uppercase ASCII character to its lowercase counterpart, returning the transformed string. The algorithm operates directly on the input string, modifying it in place, which eliminates the need for auxiliary storage.

Core Logic

The heart of the conversion is a range-based for loop that iterates over each character by reference. Inside the loop a conditional checks whether the current character falls within the ASCII range for uppercase letters ('A' to 'Z'). When the condition is true, the character is shifted to its lowercase form by adding the difference between 'a' and 'A'. The relevant fragment is:

for (auto& c : s)
    if (c >= 'A' && c <= 'Z')
        c += 'a' - 'A';

Because c is bound as a reference (auto&), the modification directly updates the original string buffer. After the loop completes, the method returns the now-lowercased string:

return s;

Edge-Case Handling

The code implicitly handles several edge conditions without additional branches:

• Empty string - The range-based loop simply does not execute, and the empty string is returned unchanged.
• Non-alphabetic characters - Characters outside the 'A'-'Z' range, such as digits, punctuation, or already lowercase letters, bypass the conditional and remain untouched.
• Mixed case strings - Only the uppercase portion is transformed; any existing lowercase characters are preserved, yielding a correctly lowercased result for any combination.
• String containing only uppercase letters - Every character satisfies the condition, so the entire buffer is transformed in a single pass.

Performance Characteristics

The algorithm exhibits a tight inner loop with a single conditional and an arithmetic operation. By operating in place, it avoids allocating extra memory, which reduces heap pressure and improves cache locality. The use of a range-based loop with a reference iterator eliminates the overhead of copying characters during iteration. The constant expression 'a' - 'A' is computed at compile time, so the addition inside the loop is a simple integer addition. This combination yields a highly efficient transformation that scales linearly with the length of the input string while maintaining minimal runtime overhead.

Imagine there is a magical box that holds a line of letters. This computer program acts like a friendly helper who walks down the line one letter at a time to check if each one is a big capital letter. If the helper finds a big letter, it gently swaps it for a matching tiny lowercase version. The helper keeps doing this task until every single letter in the line is small and cozy. Once the work is all finished, the line of letters is ready to go.

Several LLMs ranked my code quality on a scale from 1 (very poor) to 10 (outstanding):

Time: linear
As a rule of thumb, the maximum input size for one second execution time is

• factorial: up to 10 elements
• exponential: up to 20 elements
• polynomial: a few thousand elements
• loglinear: up to 100,000 elements
• linear: a few million elements
• logarithmic: a billion elements
• constant: practically unlimited

I wrote a basic crawler to find other solutions to that problem.
Obviously, I did that after I solved the problem myself.

In general, AlgoMonster and WalkCCC solutions are high-quality and always worth a look.
I haven't checked most of the other solutions, but they might be helpful as well.

And of course, the LeetCode forum is always a good place (or even the best) to look for alternative approaches.

(click on a column header to sort the table)

	author
author	solutions
www	AlgoMonster	Python	Java	C++							TS
	WalkCCC	Python	Java	C++
	leetcode.ca	Python	Java	C++	Go		C	Rust			TS
	LeetcodeHelp	Python	Java	C++
	Progiez	Python	Java	C++
github	BigEggStudy					C#
	black-shadows			C++
	chandansgowda	Python
	charles-wangkai		Java
	DimitrisJim						C
	fartem								Ruby
	halfrost				Go
	JoshCrozier									JS
	keineahnung2345			C++
	Krazune						C
	lkwq007	Python
	muhammadfarhankt				Go
	qiyuangong		Java
	ruoru							Rust
	suxrobGM					C#
	varunu28		Java
	YuriSpiridonov	Python
AlgoMonster	C++, Python, Java, TypeScript
WalkCCC	C++, Python, Java
leetcode.ca	C, C++, Go, Java, Python, Rust, TypeScript
LeetcodeHelp	C++, Java, Python
Progiez	C++, Java, Python
BigEggStudy	C#
black-shadows	C++, C++
chandansgowda	Python
charles-wangkai	Java
DimitrisJim	C
fartem	Ruby
halfrost	Go
JoshCrozier	JavaScript
keineahnung2345	C++
Krazune	C
lkwq007	Python
muhammadfarhankt	Go
qiyuangong	Java
ruoru	Rust
suxrobGM	C#
varunu28	Java
YuriSpiridonov	Python

There are a few extra pages giving you some insights into my setup:

• Tech Stack - my development environment and tools
• AI - how AI helped me create this website (NOT solving the problems)
• Progress - progression over time
• Quality - code quality