翻转字符串

public class Solution {
    /**
     * @param s : A string
     * @return : A string
     */
    public String reverseWords(String s) {
    	
    	if (s == null || s.length() == 0) {
            return "";
        }
    	
    	Stack<String> stack = new Stack<String>();
    	
    	while (s.length() != 0) {
    	    s = s.trim();  
	    	int i = s.indexOf(" ");
	    	
	    	if ( i== -1) {
	    		stack.push(s);
	    		break;
	    	}
	    	
	    	stack.push( s.substring(0, i) );
	    	s = s.substring(i, s.length());
	    	
    	}
    	
    	String str = "";
    	
    	while (!stack.isEmpty()) {
    		if(stack.size()==1)
    			str = str + stack.pop();
    		else 
    			str = str + stack.pop() + " ";
    	}
    	
    	return str;
    }
}