LeetCode Python/Java/C++/JS  >  String  >  151. Reverse Words in a String  >  Solved in Python, Java, C++, JavaScript, Go, C#, Ruby  >  GitHub or Repost

    LeetCode link: 151. Reverse Words in a String, difficulty: Medium.

    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 1:

    Input: s = "the sky is blue"

    Output: "blue is sky the"

    Example 2:

    Input: s = " hello world "

    Output: "world hello"

    Example 3:

    Input: "a good example"

    Output: "example good a"

    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.

    Follow-up: If the string data type is mutable in your language, can you solve it in-place with O(1) extra space?

    Intuition
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    1. Split the string into an array of words (need to remove empty strings)
    2. Reverse the order of the words
    3. Join the words with a single space

    Step-by-Step Solution

    1. Split the string using split(' ')
    2. Remove empty strings
    3. Reverse the word array using reverse
    4. Merge into the final string using join(' ')

    Complexity

    Time complexity

    O(N)

    Space complexity

    O(N)

    Solution in languages: Python Java C++ JavaScript Go C# Ruby

    Python # 

    class Solution:
    def reverseWords(self, s: str) -> str:
        words = [word for word in s.split(' ') if word]
        return ' '.join(words[::-1])

    Java # 

    class Solution {
    public String reverseWords(String s) {
        var wordList = new ArrayList<String>();
        var words = s.split(" ");

        for (var word : words) {
            if (!word.isEmpty()) {
                wordList.add(word);
            }
        }

        int left = 0;
        int right = wordList.size() - 1;
        while (left < right) {
            Collections.swap(wordList, left, right);
            left++;
            right--;
        }

        return String.join(" ", wordList);
    }
}

    C++ # 

    class Solution {
public:
    string reverseWords(string s) {
        istringstream iss(s);
        string word;
        vector<string> word_list;

        // 1. Extract words from the string.
        // The istringstream >> operator automatically handles
        // multiple spaces between words and leading/trailing spaces.
        while (iss >> word) {
            word_list.push_back(word);
        }

        reverse(word_list.begin(), word_list.end());

        // 2. Join the words with a single space.
        string result = "";
        result = word_list[0];
        for (auto i = 1; i < word_list.size(); ++i) {
            result += " ";
            result += word_list[i];
        }

        return result;
    }
};

    JavaScript # 

    var reverseWords = function(s) {
  const words = s.split(' ').filter((word) => word !== '');
  return words.reverse().join(' ');
};

    Go # 

    func reverseWords(s string) string {
    words := strings.Fields(s) // Fields splits on whitespace and ignores multiple spaces

    // Reverse the words
    for i, j := 0, len(words) - 1; i < j; {
        words[i], words[j] = words[j], words[i]
        i += 1
        j -= 1
    }

    return strings.Join(words, " ")
}

    C# # 

    public class Solution {
    public string ReverseWords(string s) {
        // Split into words, remove empty entries, reverse
        var words = s.Split(new[] {' '}, StringSplitOptions.RemoveEmptyEntries).Reverse();
        return string.Join(" ", words);
    }
}

    Ruby # 

    def reverse_words(s)
  s.split(' ').reject(&:empty?).reverse.join(' ')
end
    Solution in languages: Python Java C++ JavaScript Go C# Ruby

    Other languages

    Welcome to contribute code to LeetCode.blog GitHub -> 151. Reverse Words in a String. Thanks!
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