public class Solution {
    public String reverseWords(String s) {        
        if (s.length() == 0)
            return null;
        
        char [] a = s.toCharArray();
        int n = a.length;
        
        reverse(a, 0, n-1);
        reverseWords(a, n);
        return cleanSpaces(a, n);       
    }
    
    public void reverse(char [] a, int i, int j)
    {
        while(i<j)
        {
            char temp = a[i];
            a[i++] = a[j];
            a[j--] = temp;
        }
    }
    
    public void reverseWords(char [] a, int n)
    {
        int i=0, j=0;
        while(i<n)
        {
            while(i < j || i < n && a[i] == ' ') i++;
            while(j < i || j < n && a[j] != ' ') j++;
            reverse(a, i, j-1);
        }
    }
    
    public String cleanSpaces(char [] a, int n)
    {
        int i=0, j=0;
        
        while(j < n)
        {
            while (j < n && a[j] == ' ') j++;
            while (j < n && a[j] != ' ') a[i++] = a[j++];
            while (j < n && a[j] == ' ') j++; 
            if (j < n) a[i++] = ' ';
        }
        
        return new String(a).substring(0, i);
    }
    
    
}