public static String reverseString(String s){
		if(s == null){
			return null;
		}
		char[] reverseArray =  s.toCharArray();
		int lastIndex = 0;
		for(int i =0;i <= s.length();i++){
			if(i == s.length() ||  reverseArray[i] == ' '){
				reverseChar(reverseArray,lastIndex,i-1);
				lastIndex = i+1;
			}
		}
		reverseChar(reverseArray,0,s.length()-1);
		return new String(reverseArray);
	}