Reverse words of a sentence in place

Challenge

Reverse the order of the words in a sentence, in place.

Task

Given a writable C-string s of words separated by single spaces, implement void reverse_words(char *s) that reverses the order of the words within the same buffer. The characters of each word stay in their original order; only the word order changes.

Input

s: a writable, NUL-terminated string. Words are separated by single spaces, with no leading or trailing whitespace.

Output

Nothing returned; s is modified in place to hold the words in reverse order.

Example

"the quick brown fox"   ->   "fox brown quick the"
"a b"                   ->   "b a"
"hello"                 ->   "hello"
""                      ->   ""

Edge cases

• Empty string stays empty.
• A single word is unchanged.

Rules

• O(1) auxiliary memory: do not use strdup or malloc. The classic approach reverses the whole string, then reverses each word.

Why this matters

The 'reverse whole, then reverse each word' technique is a classic interview question and shows up in real text-processing pipelines that need O(1) memory.

Input format

s: writable NUL-terminated string; words separated by single spaces, no leading/trailing spaces.

Output format

Nothing returned; s holds the same words in reverse order.

Constraints

O(1) auxiliary memory. No strdup, no malloc.

Starter code

void reverse_words(char *s) { /* TODO */ }

Common mistakes

Reversing characters byte-by-byte and forgetting to also reverse each word; mishandling empty input; using strtok which mutates and loses span info.

Edge cases to handle

Empty string. One word. Two words.

Complexity

O(n) time, O(1) memory.

Background lessons

• The two-pointer technique

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