151. Reverse Words in a String

class Solution 
{
    public String reverseWords(String s)
     {
        int i = s.length() - 1;
        String ans = "";
        
        while(i >= 0) 
        {
            // Skip trailing spaces
            while(i >= 0 && s.charAt(i) == ' ')
                i--;

            // Mark the end of the word
            int j = i;

            // If we have reached the beginning, break
            if(i < 0) 
                break;

            // Skip the word
            while(i >= 0 && s.charAt(i) != ' ') 
                i--;

            // Extract the word and append it to the result
            if(ans.isEmpty()) 
            {
                ans = ans.concat(s.substring(i + 1, j + 1));
            }
            else
            {
                ans = ans.concat(" " + s.substring(i + 1, j + 1));
            }
        }
        return ans;
     }
}