Reverse Words in a String

Problem

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

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

Solution

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

Using built-in reverse method

public class Solution {
    public String reverseWords(String s) {
        if (s == null || s.length() == 0) return s;
        List<String> list = Arrays.asList(s.trim().split(" +")); //Do not forget to trim() here 
        Collections.reverse(list);
        return String.join(" ", list);
    }
}

Analysis

We can either use a StringBuilder and construct it from the reversed order of tokens
Or we can simply convert the tokens to array and then use built-in method Collections.reverse(list)
And then call String.join() to connect tokens together
Notice that we need to use trim() to take care of spaces (if not allowed, use if statement or substring())