151. Reverse Words in a String

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.

public class Solution {
    public String reverseWords(String s) {
        if (s == null || s.length() == 0) {
            return "";
        }

        String[] array = s.split(" ");
        StringBuilder sb = new StringBuilder();

        for (int i = array.length - 1; i >= 0; --i) {
            if (!array[i].equals("")) {
                sb.append(array[i]).append(" ");
            }
        }
        
        return sb.length() == 0 ? "" : sb.substring(0, sb.length() - 1);
    }
}

public class Solution {
    public String reverseWords(String s) {
        String[] words = s.split(" ");
        StringBuilder stringBuilder = new StringBuilder(); // use stringBuilder for faster but non-thread safe operations
        for (int i = words.length - 1; i >= 0; i--) {
            if (words[i].length() > 0) {
                stringBuilder.append(words[i]);
                stringBuilder.append(" "); // don't use string concatenation since it's slower.
            }
        }
        return stringBuilder.toString().trim();
    }
}

/**
 * @param {string} s
 * @return {string}
 */
var reverseWords = function(s) {
    let arr = s.split(" ").filter(elem => elem !== "");
    return arr.reverse().join(" ");
};