138. Reverse Words in a String

Splitting on whitespace naturally drops leading, trailing, and repeated spaces and yields the words as a list. Reverse that list and join it back with single spaces.

Idea

The messy part of this problem is the spacing — leading, trailing, and multiple internal spaces. Splitting on runs of whitespace cleanly extracts just the words, discarding all the extra spaces. Then reversing the word list and joining with a single space produces the exact required format.

Algorithm

1. Split s on whitespace into a list of words (empty tokens dropped).
2. Reverse the list.
3. Join with a single space and return.

Worked example: " the sky is blue "

• Split → ['the', 'sky', 'is', 'blue'] (all extra spaces gone).
• Reverse → ['blue', 'is', 'sky', 'the'].
• Join → "blue is sky the".

Complexity

• Time: O(n).
• Space: O(n) for the word list and output.

class Solution {
    public String reverseWords(String s) {
        String[] words = s.trim().split("\\s+");
        StringBuilder sb = new StringBuilder();
        for (int i = words.length - 1; i >= 0; i--) {
            sb.append(words[i]);
            if (i > 0) {
                sb.append(" ");
            }
        }
        return sb.toString();
    }
}