151. Reverse Words in a String

Q: Why does Microsoft like the reverse-then-reverse-each-word trick?

It generalizes to other in-place rotation problems and shows you understand that two reversals compose to a rotation of word order. The same idea solves array rotation in O(1) space.

Q: Is split+reverse+join actually accepted?

Yes for correctness, but the in-place follow-up is almost guaranteed. Write the clean version first, then sketch the in-place version unprompted.

mediumAsked at Microsoft

Reverse Words in a String is Microsoft's go-to string-manipulation question. The clean two-liner (split, reverse, filter, join) is acceptable but interviewers grade highly on the in-place O(1) version that reverses the whole string then reverses each word.

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-19

Source citations

Public interview reports confirming this problem appears in Microsoft loops.

Glassdoor (2026-Q1)— Microsoft Windows/Office org onsite reports list Reverse Words as a recurring 20-minute string medium.
Blind (2025-11)— Microsoft new-grad and L60 reports flag Reverse Words with the in-place follow-up.

Problem

Given an input string s, reverse the order of the words. A word is defined as a sequence of non-space characters. The words in s will be separated by at least one space. Return a string of the words in reverse order concatenated by a single space. Note that s may contain leading or trailing spaces or multiple spaces between two words. The returned string should only have a single space separating the words. Do not include any extra spaces.

Constraints

1 <= s.length <= 10^4
s contains English letters (upper-case and lower-case), digits, and spaces ' '.
There is at least one word in s.

Examples

Example 1

Input

s = "the sky is blue"

Output

"blue is sky the"

Example 2

Input

s = "  hello world  "

Output

"world hello"

Explanation: Your reversed string should not contain leading or trailing spaces.

Example 3

Input

s = "a good   example"

Output

"example good a"

Explanation: Reduce multiple spaces between two words to a single space.

Approaches

1. Split + filter + reverse (idiomatic)

Split on whitespace, drop empty tokens (from collapsed multi-space), reverse the array, join with a single space.

Time: O(n)
Space: O(n)

function reverseWords(s) {
  return s.trim().split(/\s+/).reverse().join(' ');
}

Tradeoff: Cleanest one-liner. Acceptable at most companies, but Microsoft will ask for the in-place version on a follow-up because their interview rubric specifically tests whether you can manipulate a string buffer without allocating a new one.

2. Two-pointer parse without regex

Walk the string left to right, collect non-space chars into a word, push to a result array when a space is hit, reverse at the end.

Time: O(n)
Space: O(n)

function reverseWords(s) {
  const words = [];
  let i = 0;
  while (i < s.length) {
    while (i < s.length && s[i] === ' ') i++;
    if (i === s.length) break;
    let j = i;
    while (j < s.length && s[j] !== ' ') j++;
    words.push(s.slice(i, j));
    i = j;
  }
  return words.reverse().join(' ');
}

Tradeoff: Same complexity as the split version but no regex — useful when the interviewer says 'pretend you can't use split.' Makes the leading/trailing/multi-space handling explicit, which Microsoft graders like.

3. Reverse-then-reverse-each-word (in-place mental model)

Reverse the entire string, then reverse each word in place. Final pass to collapse extra spaces. With a char array this is O(1) extra space.

Time: O(n)
Space: O(1) for char array (true in C/C++ where strings are mutable)

function reverseWords(s) {
  const chars = s.trim().replace(/\s+/g, ' ').split('');
  function reverse(l, r) {
    while (l < r) { [chars[l], chars[r]] = [chars[r], chars[l]]; l++; r--; }
  }
  reverse(0, chars.length - 1);
  let start = 0;
  for (let i = 0; i <= chars.length; i++) {
    if (i === chars.length || chars[i] === ' ') {
      reverse(start, i - 1);
      start = i + 1;
    }
  }
  return chars.join('');
}

Tradeoff: The 'true' in-place version. In JavaScript strings are immutable so we still allocate, but the algorithm is what Microsoft is grading — describe it as the C/C++ in-place algorithm and they're happy.

Microsoft-specific tips

Microsoft will ALWAYS ask 'can you do this in-place?' as a follow-up. Lead with the clean split-reverse-join answer to confirm correctness, then immediately offer 'in C this would be reverse the whole buffer then reverse each word, O(1) extra space.' Saying that sentence unprompted earns the optimal-solution bonus.

Common mistakes

Forgetting to trim leading/trailing spaces in the output.
Forgetting to collapse multiple internal spaces.
Using s.split(' ') instead of s.split(/\s+/) — the former returns empty strings for collapsed spaces.

Follow-up questions

An interviewer at Microsoft may pivot to one of these next:

Reverse Words in a String II (LC 186) — same problem with a char array as input, asking for in-place.
Reverse Words in a String III (LC 557) — reverse each word in place, don't change word order.
What if you had to do this with arbitrary Unicode (combining characters, surrogates)?

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Output

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FAQ

Why does Microsoft like the reverse-then-reverse-each-word trick?

It generalizes to other in-place rotation problems and shows you understand that two reversals compose to a rotation of word order. The same idea solves array rotation in O(1) space.

Is split+reverse+join actually accepted?

Yes for correctness, but the in-place follow-up is almost guaranteed. Write the clean version first, then sketch the in-place version unprompted.

Free learning resources

Curated free links for this problem.

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