151. Reverse Words in a String (Medium)

Given an input string, reverse the string word by word.

For example,
Given s = "the sky is blue",
return "blue is sky the".

Update (2015-02-12):
For C programmers: Try to solve it in-place in O(1) space.

click to show clarification.

Clarification:

• What constitutes a word?

  A sequence of non-space characters constitutes a word.

• Could the input string contain leading or trailing spaces?

  Yes. However, your reversed string should not contain leading or trailing spaces.

• How about multiple spaces between two words?

  Reduce them to a single space in the reversed string.

    /**
     * O(n); O(n)
     * Trim input string and split it with a space. Use StringBuilder to store result.
     * Traversal backwards and add each non-empty word to result.
     */
    public String reverseWords(String s) {
        if (s == null || s.length() == 0) {
            return "";
        }

        s = s.trim();
        StringBuilder res = new StringBuilder();
        String[] words = s.split(" ");
        for (int i = words.length - 1; i >= 0; i--) {
            if (!words[i].equals("")) {
                res.append(words[i]);
                if (i != 0) {
                    res.append(" ");
                }
            }
        }
        return res.toString();
    }