Camel case to snake case

Convert camelCase strings to snake_case with a single function.

strings case-conversion parsing

Write a function `camel_to_snake(s: str) -> str` that converts a camelCase string to snake_case. The input contains only letters and digits, may start with a capital letter (PascalCase), and may include consecutive uppercase letters (acronyms) and digits. Rules:
- Insert an underscore before each uppercase letter that is followed by a lowercase letter or digit (e.g., "FooBar" -> "foo_bar").
- Insert an underscore between a lowercase/digit and an uppercase letter (e.g., "fooBar" -> "foo_bar").
- For acronyms: insert an underscore before the last uppercase letter of a consecutive uppercase sequence if it is followed by a lowercase letter (e.g., "HTTPServer" -> "http_server").
- Digits are treated as lowercase-like: transitions between lowercase/digit and uppercase trigger underscores.
- Convert all letters to lowercase.
- The output should never start or end with an underscore.

Examples:
- `camel_to_snake("helloWorld")` → `"hello_world"`
- `camel_to_snake("HelloWorld")` → `"hello_world"`
- `camel_to_snake("HTTPServer")` → `"http_server"`
- `camel_to_snake("foo2Bar")` → `"foo2_bar"`

Constraints

Input length: 1 ≤ len(s) ≤ 1000. Input contains only ASCII letters (A-Z, a-z) and digits. The function must handle large inputs efficiently (O(n) time, O(n) space).

Example

>>> camel_to_snake("helloWorld")
'hello_world'
>>> camel_to_snake("HelloWorld")
'hello_world'
>>> camel_to_snake("HTTPServer")
'http_server'
>>> camel_to_snake("foo2Bar")
'foo2_bar'

10 points ~15 min

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Hints

Iterate through characters and check transitions between lowercase/digit and uppercase.

Track when a sequence of uppercase letters ends before a lowercase letter to insert an extra underscore.

Consider how acronyms like 'HTTPServer' should become 'http_server' not 'h_t_t_p_server'.

Test Results

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