Problem statement

Given an input string s, reverse the order of the words.

A word is a maximal run of non-space characters. The words in s may be separated by more than one space. The returned string must have the words in reverse order joined by a single space, with no leading or trailing spaces.

Example 1

Input: s = "the sky is blue"
Output: blue is sky the

Example 2

Input: s = "  hello world  "
Output: world hello
Explanation: The surrounding spaces are dropped.

Example 3

Input: s = "a good   example"
Output: example good a
Explanation: Runs of spaces collapse to one.

Constraints

• 1 <= s.length <= 60
• s consists of lowercase English letters and spaces.
• There is at least one word in s.

Hints for Reverse Words in a String

1. Hint 1

   Split on whitespace and discard the empty pieces that runs of spaces produce.

2. Hint 2

   Reverse the list of words, then join with a single space.

3. Hint 3

   The in-place variant reverses the whole string, then reverses each word again.

How to solve Reverse Words in a String

Tokenise on whitespace — which also discards the empty pieces produced by multiple spaces — reverse the token list, and join with single spaces.

Approach

1. Split the string on runs of whitespace, dropping empty pieces.
2. Reverse the list of words.
3. Join with a single space.

Why it works

Splitting on whitespace runs normalises the spacing for free: leading, trailing and repeated spaces all vanish, leaving only the words. Reversing and rejoining then produces exactly the required format.

Complexity

• Time — O(n)
• Space — O(n)

Pitfalls

• Splitting on a single space keeps empty strings for repeated spaces; either filter them or split on runs.
• The O(1)-extra-space variant reverses the whole array, then reverses each word in place, then compacts the spaces.

Reference solution

Python

def reverseWords(s: str) -> str:
    return " ".join(reversed(s.split()))

Java

public static String reverseWords(String s) {
    String[] pieces = s.split(" ");
    List<String> words = new ArrayList<>();
    for (String w : pieces) {
        if (w.length() > 0) words.add(w);
    }
    Collections.reverse(words);
    return String.join(" ", words);
}

C++

string reverseWords(string s) {
    vector<string> words;
    string cur = "";
    for (char ch : s) {
        if (ch == ' ') {
            if (!cur.empty()) { words.push_back(cur); cur = ""; }
        } else {
            cur += ch;
        }
    }
    if (!cur.empty()) words.push_back(cur);
    string out = "";
    for (int i = (int) words.size() - 1; i >= 0; i--) {
        if (!out.empty()) out += " ";
        out += words[i];
    }
    return out;
}

JavaScript

var reverseWords = function(s) {
    const words = s.split(" ").filter(function(w) { return w.length > 0; });
    words.reverse();
    return words.join(" ");
};

Also on the editorial tab: C, C#, Go, Kotlin, PHP, Ruby, Rust, Swift, TypeScript.

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