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

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

click to show clarification.

• What constitutes a word?
  A sequence of non-space characters constitutes a word.
• Could the input string contain leading or trailing spaces?
  Yes. However, your reversed string should not contain leading or trailing spaces.
• How about multiple spaces between two words?
  Reduce them to a single space in the reversed string.
• 方法就是先將整個string翻轉，然後再將多餘空格變為一個空格，在將其中每個單詞翻轉即可。
• 我為了避免調整空格的繁瑣移動操作，就直接另建一string，保留每個word的翻轉結果，最後將其賦值給s即可
  

  class Solution {
  public:
      void reverseWords(string &s) {
          if( s.empty() )
  			{ return; }
  			reverse( s.begin(),s.end() );
  			string::iterator left = s.begin(),right = s.begin();
  			enum{ OFF,ON } state;
  			state = OFF;
  			string s1;
  			for( string::iterator i = s.begin();i != s.end();i++ )	{
  				if( *i != ' ' )	{	
  					if( !state )	{
  						left = i;
  						state = ON;
  					}
  				}
  				else if( *i == ' ' )	{
  					if( state )	{
  						right = i;
  						s1 += reverse( left,right ) + ' ';
  						state = OFF;
  					}
  				}
  			}
  			if( s[s.size() - 1] != ' ')
  			    s1 += reverse( left,s.end() );
  			if( s1[s1.size() - 1] == ' ')
  			    s1.erase( s1.end() - 1 );
  			s = s1;
  	}
  	string reverse( string::iterator left,string::iterator right )	{
  			char c;
  			for( string::iterator it1 = left,it2 = right - 1;it1 < it2;it1++,it2-- )	{
  				c = *it1;
  				*it1 = *it2;
  				*it2 = c;
  			}
  			string str( left,right );
  			return str;
  		}
  };