Reverse Words in a String

 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"

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

Example 3:

Input: 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:

1 <= s.length <= 104

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?

class Solution {

    public String reverseWords(String s) {

        // Step 1: Trim leading/trailing spaces

        s = s.trim();

        // Step 2: Split the string by one or more spaces

        String[] word = s.split("\\s+"); // ["a", "good", "example"]

        // Step 3: Reverse the array

        int left = 0;

        int right = word.length - 1;

        while (left < right) {

            String temp = word[left];

            word[left] = word[right];

            word[right] = temp;

            left++;

            right--;

        }

        // Step 4: Join the reversed words with single space

        return String.join(" ", word);

    }

}