Reverse Words in a String

Difficulty: Medium

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.

Example

Input: s = "  the sky  is blue  "
Output: "blue is sky the"
Explanation: After reversing the word order and trimming extra spaces: "blue is sky the".

Brute-force approach

Split the string by spaces, filter out empty entries, reverse the list of words, and join with a single space.

Steps

  1. Split the string by whitespace
  2. Filter out empty strings
  3. Reverse the word list
  4. Join with single space

Time complexity: O(n) · Space complexity: O(n)

Trade-offs

  • Uses O(n) extra space for the word list
  • Relies on built-in split — interviewers may ask for in-place
  • Some languages create intermediate string copies during split

Optimal approach

Key insight: Reversing the entire string puts words in the right order but with reversed characters. Reversing each word individually fixes the characters.

For an in-place approach: (1) reverse the entire string, (2) reverse each individual word, (3) clean up extra spaces. This avoids the word list allocation.

Steps

  1. Trim leading/trailing spaces, collapse multiple spaces
  2. Reverse the entire character array
  3. Iterate through, reverse each word individually
  4. Return the result string

Time complexity: O(n) · Space complexity: O(1)