> For the complete documentation index, see [llms.txt](https://chunhthanhde.gitbook.io/leetcode-top-interview/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://chunhthanhde.gitbook.io/leetcode-top-interview/topic-1-array-string/021-reverse-words-in-a-string.md).

# 151. Reverse Words in a String 🔄

**Difficulty**: `Medium` - **Tags**: `String`, `Two Pointers`

[LeetCode Problem Link](https://leetcode.com/problems/reverse-words-in-a-string/description/)

## Description

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.

## Examples

**Example 1:**

Input:

```java
s = "the sky is blue"
```

Output:

```
"blue is sky the"
```

**Example 2:**

Input:

```java
s = "  hello world  "
```

Output:

```
"world hello"
```

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

**Example 3:**

Input:

```java
s = "a good   example"
```

Output:

```
"example good a"
```

Explanation: You need to reduce multiple spaces between two words to a single space in the reversed string.

## Constraints

* The input string `s` contains only printable ASCII characters.
* `s` does not contain any leading or trailing spaces.
* The words in `s` will be separated by a single space.

## Solution 💡

To solve this problem, we can split the string into words, remove any extra spaces, and reverse the list of words. Finally, we join them back into a string with a single space separating the words.

### Java

```java
class Solution {
    public String reverseWords(String s) {
        // Trim the string and split by spaces
        String[] words = s.trim().split("\\s+");
        // Initialize a StringBuilder to store the reversed words
        StringBuilder reversed = new StringBuilder();
        
        // Traverse the words array in reverse order and append them to the StringBuilder
        for (int i = words.length - 1; i >= 0; i--) {
            reversed.append(words[i]);
            if (i > 0) {
                reversed.append(" ");
            }
        }
        
        // Convert StringBuilder back to String
        return reversed.toString();
    }
}
```

### Time Complexity ⏳

* **O(n)**: The time complexity is linear, where `n` is the length of the input string.

### Space Complexity 💾

* **O(n)**: The space complexity is also linear, as we are storing the reversed words in a new string.

You can find the full solution [here](https://github.com/ChunhThanhDe/Leetcode-Top-Interview/blob/main/Topic%201%20Array%20-%20String/021%20Reverse%20Words%20in%20a%20String/Solution.java).


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://chunhthanhde.gitbook.io/leetcode-top-interview/topic-1-array-string/021-reverse-words-in-a-string.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.